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Preferred Sanding Grit for Filler/Primer?

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TangoJuliet

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I've been pretty busy building LPR/MPR kits as a BAR, and that now also involves spraying and sanding filler/primer - something I never considered or even knew about when I was younger. I've been using Rust-Oleum Gray Filler Primer p_1000655181.jpgand I've been pretty pleased with it, but I'm not sure about the sandpaper I've been using. I've been using 3M 400 grit 051131922495.jpg, but it seems to gum up really fast, to the point that it seems to be polishing the primer rather than removing it. I've been afraid to use a more aggressive grit, but maybe I need to.

Looking for suggestions please.
 

BDB

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I'm no expert, but I've been using the 220 grit flexible stuff from 3M for nearly everything lately, including sanding the same filler primer that you are using. I like it because the sheets are durable and easy to wash out if they get gummed up.
 

T-Rex

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You didn't mention it, so I'll throw it out here... Start with something a little coarser and work up to the 400. 120 -> 220 -> 400. Don't go crazy with the coarse or there will be nothing left.
The idea is you remove the majority of the high spots and then just polish with the 400.

The other thing that leads to clogging of abrasive is too much pressure. The dust has to be able to migrate out. With too much pressure it can't go anywhere and fills the gaps in the abrasive.
 

KenRico

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I use the Rusto Filler Primer from the Auto section at Wally World ... and sand it with 220 grit .

After I have a uniform smooth surface I do a couple of 'finish primer' coats and wet sand them with 320 grit . The White finish primer I use is the Krylon white primer (not 2x ) from the Wally World spray paint section .

Kenny
 

Nytrunner

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I'll have to give that filler primer a try. May cut down on the amount of wood filler or spot putty I need to apply.

For sanding primer coats, I've just been starting with some damp 220, then a couple passes with damp 500. I've found that I can keep the paper pretty clear by swishing it back and forth in water, shaking most of it off, then going right back to sanding. (of course this requires paper that can handle moisture)
 

Steve Shannon

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I think one of the biggest mistakes people make when sanding is to try too hard to avoid coarse grit paper. As a result it becomes a long ordeal. Depending on what I'm doing I'll start as low as 80, then 160, finally finishing with 240 or higher. Each grit increment I try to get rid of half the depth of sanding marks from the previous grit.
 

TangoJuliet

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Thanks everybody. On balsa fins I use AeroGloss Balsa Filler prior to spraying the entire rocket with the Rust-O primer, to fill as much of the grain as I can beforehand. I guess I need to start with at least a 220 grit. I'll still avoid using it wet though as my BT's are all paper and I don't want to risk damage.
 

boatgeek

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One more thing--your finish coat over the top of everything else will fill in a lot of the sanding scratches. Going to a grit finer than the paint will fill is probably more work than you need to do. I don't go much past 220 for primer, but I'm also OK with a less perfect paint job. If you want to get really excited, you could prime a piece of plywood and sand different areas with different grits. Coating over with your finish coat will show which ones give you the finish surface of your dreams.
 

neil_w

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I start with 220 and sand gently to remove the bulk. Once I get close to the end, I switch to 400 for "finishing".

In my experience the 220 does not clog. The 400 does, but I'm left with much less to sand after the 220.
 

mccordmw

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I've gotten away from wet sanding primer layers. It gets too gloppy and soaks up the water.

I'm rather spastic in switching out methods to see if something works better. I currently use the following steps:

First coat is high-build primer: https://www.amazon.com/gp/product/B000B6DG7M/?tag=skimlinks_replacement-20
Sand with 220 grit. This primer feels very soft and porous to me. It sands down very fast!
Switch to regular sandable primer and still use 220 grit.
Once I've got everything smooth enough for my liking, the last primer coat is 400 grit.

I do like to alternate red and gray primer colors so I can easily tell what I've primed. The final primer color depends on my rocket paint color. I want a primer color similar to the paint color so there's no color bleed through.
 
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ascastil

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Hows that rustoleum filler primer work on filling in the spirals or is some other filler still being used. Might be worth it to pick up a can if it helps with that chore.
 

mccordmw

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Hows that rustoleum filler primer work on filling in the spirals or is some other filler still being used. Might be worth it to pick up a can if it helps with that chore.
It will minimize them greatly in about 4 coats. But that's a pricey substitute vs CWF. It's still no substitute for CWF. I find that I don't need to be super careful with CWF, though. I just slather it on the seams and scrape it all off with a plastic putty knife. Scrape it all off the tube, leaving behind a bit in the seams. The high build primer will do the rest then.
 

Nathan

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Hows that rustoleum filler primer work on filling in the spirals or is some other filler still being used. Might be worth it to pick up a can if it helps with that chore.
It takes a lot of filler primer coats to completely hide spiral marks. You'll save a lot of time and effort by filling with CWF first, then one or two coats of sanded filler primer and it will be perfect.
 

ksaves2

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Ummm, Painting trick I've been told. Don't sand the primer to a super fine polish. Reason? the color coat paint may not "stick" very well to the highly polished surface. Fellow once mentioned to get good coverage and sand between coats but the last coat
spray at a distance so one gets a surface texturing effect with the primer droplets. Let dry and you'll feel a gritty texture. Then shoot the color. If you're going for lacquer over the color coat, that's when you do the wet sanding to
even out the color and then shoot the clear gloss lacquer (if going for a gloss look). Then the fun begins with wet sanding the lacquer with the "superfine" sand paper 1000, 1500, 2000 etc. Then rubbing compound, next with polishing compound
and then a good car wax. If one does all the above, get ready for a heart attack with the first ding.
Kurt
 

mpitfield

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I am going to go the other way.

I regularly wet sand with 400 and even 600 without any issues with adhesion. That is for FW/FG and plastic, but not on paper tubes. Paper tubes I dry sand to 320.

I find wet-sanding superior to dry sanding because I sand under running water or from a bucket, and in doing so the water flushes away the debris as I sand. 600 may sound smooth but it is not for any of the paints that I have used, and they include Rusto2x, Rusto reg, Dupli-color Acrylic Enamel in a can, Dupli-color HVLP Paint Shop 123 system (lacquer based automotive), Plutonium (lacquer based automotive), Krylon ColorMaster, Krylon Camouflage Matte, and I am likely forgetting a few.

The bigger issue I have found is oils transferring from my hands during the sanding process into the primer, which is why I now wear gloves while sanding and handling the rocket during the painting process. I have also found that the more prep work I do, and I consider filling and priming "prep work', the less painting and mucking around trying to get a good finish, so it pays off. Plus most primers I have priced out are less expensive than base and top coats.
 

Woody's Workshop

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180 cuts pretty fast prior to priming. Finish with 220 before priming.
2 good coats of primer is all you should need.
if needed, you can dry sand primer with 220 for recoating.
400 wet sanding for pre-painting is preferred.
I keep a container of warm water at hand, dipping and rinsing the sandpaper.
Occasionally wiping away residue with a paper towel or rag. Usually an old sock or piece of old T-Shirt.
Wipe down with a wet clean coat and let dry over night.
Finish spray the next day.
I always coat the inside of the body tube with TiteBond II.
Adds strength to the tube, prevents moister wicking when wet sanding, and protects the tube on ejection charges.
That's the way I do things.
I use to do custom automotive painting. Less is more.
Meaning, the less paint build up, the lesser chances are of chipping and cracking down the road.
If you clear coat, make sure it is UV protective, or it will yellow over time, even if it isn't subjected to UV rays.
And coat of automotive paste wax never hurt a good paint job, or even a so-so paint job.
 

Micromeister

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Over the last 40 years as a Sign Painter I've leaned a bunch of things about sanding Primers. Doesn't matter who's or the brand..Rattle Can or Premium Grade Automotive primers. For best adhesion of paint to the primer start with 220 or 240 grit to knock down the hills, switch to 320 -360 for the final sanding. We want those microscopic hills and valleys to help the paint grab onto the 360grit sanded primer. Any further sanding does not increase the smoothness of the finished painted product or help the adhesion of the paint to the primers.

400 to 30,000 grit sandpapers are specifically made for Wet Sanding the final, finished top color or clear coat paints NOT PRIMER.
 

markkoelsch

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Hows that rustoleum filler primer work on filling in the spirals or is some other filler still being used. Might be worth it to pick up a can if it helps with that chore.
Fillers is a topic unto itself.

Some folks may agree or disagree. For phenolic and thicker tube spirals I like elmers wood filler.

For composites I like Polyfibers Superfil
 

markkoelsch

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Over the last 40 years as a Sign Painter I've leaned a bunch of things about sanding Primers. Doesn't matter who's or the brand..Rattle Can or Premium Grade Automotive primers. For best adhesion of paint to the primer start with 220 or 240 grit to knock down the hills, switch to 320 -360 for the final sanding. We want those microscopic hills and valleys to help the paint grab onto the 360grit sanded primer. Any further sanding does not increase the smoothness of the finished painted product or help the adhesion of the paint to the primers.

400 to 30,000 grit sandpapers are specifically made for Wet Sanding the final, finished top color or clear coat paints NOT PRIMER.
30000 grit? That has to be smoother that paper? What would the point be in something like this?
 

hcmbanjo

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After on application of CWF and sanding I then use just use one coat, a medium application of Duplicolor Filler/Primer and sanding.
For me, I go light with 220 grit to take down the filler primer.
You've got to go light, otherwise you'll see sandpaper scratches when the color goes on.
The idea with the filler/primer is to leave some in the low spots filling in any remaining tube seams or balsa grain.
Color coats follow.

Here's my Filling / Painting schedule:
https://modelrocketbuilding.blogspot.com/2013/04/my-finishing-schedule-tips-part-1.html?m=0
http://modelrocketbuilding.blogspot.com/2013/04/my-finishing-schedule-tips-part-2.html

In the end, there's no right or wrong way, whatever works for you.

FinishSched 1.jpg FinishSched 2.jpg HoJo Fins.jpg
 

rharshberger

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30000 grit? That has to be smoother that paper? What would the point be in something like this?
They are extremely smooth the super/micro fine papers are sometimes referred to as Lapping films, and the only ones I have seen are an abrasive bonded to a sheet of plastic. Honestly the 2000grit wet/dry sandpapers are about as fine as we usually will have a need for, polishing creams and pastes will do the rest and they are finer than 2000grit afaik. I use the lapping films in gunsmithing, since they remove VERY small amounts of metal (once metal is taken off its very hard and time consuming to put back). I usually use 100 and 150 grit papers on my filler primer then 220 on the final primer, then progressively finer papers between color coats up to 600grit, no desire to polish a finish just make the rocket look good from 10'.
 

mkadams001

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Over the last 40 years as a Sign Painter I've leaned a bunch of things about sanding Primers. Doesn't matter who's or the brand..Rattle Can or Premium Grade Automotive primers. For best adhesion of paint to the primer start with 220 or 240 grit to knock down the hills, switch to 320 -360 for the final sanding. We want those microscopic hills and valleys to help the paint grab onto the 360grit sanded primer. Any further sanding does not increase the smoothness of the finished painted product or help the adhesion of the paint to the primers.

400 to 30,000 grit sandpapers are specifically made for Wet Sanding the final, finished top color or clear coat paints NOT PRIMER.
This is absolutely correct.

For rockets I go with 220 then 320 then paint.
The primer I use will fill the body tube spirals. (Duplicolor)
Don't use wet/dry sandpaper. It is more expensive and better suited for wet sanding. It will clog very quickly when used dry.

To the OP, just change the sandpaper you are using to dry instead of wet/dry.

I'm a sign guy too for close to 40 years.
 

TangoJuliet

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For rockets I go with 220 then 320 then paint.
The primer I use will fill the body tube spirals. (Duplicolor)
Don't use wet/dry sandpaper. It is more expensive and better suited for wet sanding. It will clog very quickly when used dry.

To the OP, just change the sandpaper you are using to dry instead of wet/dry.
I shall make those changes. :wink:
 

mpitfield

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One practical reason to use wet over dry; less mess. No mask required, no dust to sweep up, no need to wash your hair, clothes, etc.
 

Micromeister

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30000 grit? That has to be smoother that paper? What would the point be in something like this?
Actually 30,000 grit is used in microscopy, polishing samples for use under an electron microscope. My parents were in this field at NRL labs. Yes I have sanding "actually Polishing papers from 8,000 to 30,000grit that are occasionally used on super fine scale model finishes.
 
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