Sanding Primer Coats

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calambert

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I have read a number of threads claiming that a final primer coat should not be sanded with grit finer than 400 or 600, or your coat of paint won't adhere well. Given that lacquer adheres by melting into the proceeding coat, wouldn't it be OK to lightly sand the final coat of primer on the BT or nosecone with 800, 1000, or even 1500 grit prior to applying a coat of lacquer spray paint?
 
I have read a number of threads claiming that a final primer coat should not be sanded with grit finer than 400 or 600, or your coat of paint won't adhere well.

I don't believe that to be a fact. I've wet sanded primer with 1000 grit, the paint sticks just fine and turns out looking like glass.

I know 'hot-rod' body men/painters that actually -buff- the primer. The smoother the base coat, the smoother the top coat...

I have wet sanded up to 2000 grit in between color coats. Nothing like making a spray bomb paint job looking like it was done with a HVLP ;)
 
I don't believe that to be a fact. I've wet sanded primer with 1000 grit, the paint sticks just fine and turns out looking like glass.

I know 'hot-rod' body men/painters that actually -buff- the primer. The smoother the base coat, the smoother the top coat...

I have wet sanded up to 2000 grit in between color coats. Nothing like making a spray bomb paint job looking like it was done with a HVLP ;)


I have to disagree strongly with jeffs response;
As a professional painter we are taught by ALL the major paint manfacuters that final primer coats should be fine DRY sanded with no more the 360-400 grit sand paper. Because primer is by its chemistry porous wet sanding can and will cause water to migrate all the way to the substrate (on cars bare Metal - on most model rocket cardboard) which can cause Rust or delamination of the body tube). Wet sanding is a technique intended ONLY for use on the Finial Top coat and/or Clear Coats.

The reason for not sanding primer beyond 400girt is the surface needs a little tooth two allow the paint to adhear better to the micro peaks & valleys left by the grit.
Remember the ONLY thing that keeps paint on any surface is Surface Tension (the lack of air between substrate and paint film). Yes, the platelets of pigment in paint overlap and interlock forming a dry film but they do NOT melt into the underlayer. It is the thinners and vehicle that cause damage to the underlaying layers.
 
I understand the wet sanding primer issue and I will only dry sand primer. My question just pertains to how fine a grit can be used for dry sanding primer prior to applying lacquer. I have read tons of painting threads here and have have not been able to find a single one that discusses someone having problems with lacquer adhering to primer because they sanded the primer with too fine a grit. I'm beginning to think this idea that you should not sand primer with anything finer than 400 or 600 grit prior to spraying it with lacquer (so there is "tooth" to hold the lacquer) may be a myth that has been often repeated). Searching google I find most auto painting instructions call for sanding primer with 1000 grit prior to spraying. (They of course wet sand the primer, which I won't). Has anyone ever heard of problems with dry sanding primer with 1000 or 1500 grit prior to applying a lacquer coat? If you know of a thread that discusses issues with this, I'm really interested in finding out.
 
I've done 1000 grit on auto spray gun primer prior to shooting base coat and then 1500, 2000 on base coat before clear. Never needed to ever cut my clear coat or polish...ever. I take Into consideration the materials used, the area, the prep, the colors, temp, drying time and surface. Only way to figure out what works for you and to find what you are good at is trial and error and experience.

I say do what you want and see what works and how it come out. That's what I cut my teeth doing and you will be surprised at how good it comes out. Take into consideration the advice and general practices that are out there and make them your own.
 
Also to wet sand you don't need to fully wet out the area you are sanding. Lately I've been using a spray bottle with water and about 2oz of simple green to spray the sand paper, sand my small areas at a time, wipe the sand paper off on a towel, the. Wipe the area Sanded with a damp towel and move on. I also don't use much pressure and let the paper do its thing especially if I'm not using a block.

For the final wet sand spray the paper again and go over it once. Don't need a bucket and wetting out the whole area and all that... you can but I save that for painting fibglass/carbon. Let it dry for a day before you spray and wipe with wax/grease cleaner, wipe on, wipe off.
 
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