painting/finishing question

skybuster said:

hey everyone,
I have so far built a grad total of two rockets, so I am brand new into rocketry as you could expect. With both of mine, (A bib Bertha and an Astron Elliptic II), I have used spray paint on both, and I have had an awkward finish on some parts. The awkward finish looks like a hard boiled egg shell when you crack it; small, jagged, geometrical pieces. I was just wondering if anyone knows what can cause this, and if there's any suggestions/methods to correct/fix it?

thanks for the help

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SteveF said:

did you use more than one coat? if so you probably used enamel paint and reapplied the second coat too soon - does the paint say second coat in an hour or after 48 hours? or something similar. with that paint if you apply too soon that is what happens, even though the first coat looks diry it really isnt. if this is the type of paint you have (probably is as that is what is most available these days thanks to EPA)then follow the recoat times carefully and going even longer between coats is even better, especially if you are going to mask parts for a second color

If that isnt the case other things can cause that like grease or oils on the surface or high humidity

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skybuster said:

hey everyone,
I have so far built a grad total of two rockets, so I am brand new into rocketry as you could expect. With both of mine, (A big Bertha and an Astron Elliptic II), I have used spray paint on both, and I have had an awkward finish on some parts. The awkward finish looks like a hard boiled egg shell when you crack it; small, jagged, geometrical pieces. I was just wondering if anyone knows what can cause this, and if there's any suggestions/methods to correct/fix it?

thanks for the help

Click to expand...

boomtube said:

I have encountered a couple of plastic nosecones and a solid tube connector from two different companies, though I assume they were molded at the same Chinese factory and supplied to both kit companies, that have defied any and all attempts to get a good paint finish.

The plastic, as it arrives, feels “waxy” and it doesn’t wash-off. I shot it with Krylon primer and two hours later it was still sticky. In fact it never dried I had to use paint stripper to remove it so as to try, try, try, try, try, try and try again with various types/brands of paints and primers.

Eventually it went into the trash can and I replaced them both with balsa offerings from Semroc and BMS.

I hope this is not indicative of future plastic parts in our kits.

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MarkII said:

Pay attention to the recoat times stated on the label. If the manufacturer says to recoat within an hour or after 48 hours, they mean it; stay within those time limits. Otherwise you will get "lifting" and wrinkling, sometimes referred to as "orange peel." Each coating of paint goes through various phases as it dries, and at some points it is not strongly bonded to the surface. A subsequent coat, applied at the wrong time, can pull up and shrink the underlying coat. This shrinking action produces the network of wrinkles that you saw. If you apply another coat within the first block of recoat time, you'll get the second coat on before the first coat has entered this delicate stage. Both coats will be in the same phase of the drying process, so they will dry together, more or less as a single coat. If you wait until the second recoat interval, then you will be applying it after the first coat has fully dried and adhered to the substrate. At that point you'll be applying paint to a hard, dry surface.

Keep in mind that one of the constituents of spray paint is solvent, and that gets applied to the surface along with the rest of the paint components. So in a sense you are not just spraying the surface with paint, but also with solvent. If you apply a solvent to fresh, still wet paint, no problem; the solvent from the second coat just gets added to the solvent that is still there from the first coat and the two evaporate away together. If you apply a solvent to a coat of fully dried paint, it will have little visible effect on that first coat, because the dried paint by then is strongly bonded to the surface and is pretty resistant to attack by the spray's solvent. It is when you apply solvent to partially-cured paint that you have problems. There are more technical explanations of the phenomenon that involve the chemistry of spray paint and the physics of drying, but this is the overall picture in a nutshell.

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luke strawwalker said:

Interesting... I was always taught that "orange peel" was the effect of getting too far away from the surface and the paint partially drying before it hit the surface, leading to the paint not flowing out properly and ending up rough, like the peel of an orange... get REALLY too far away and you get 'dry spray' where the paint surface not only doesn't flow out, but gets downright 'pebbly' from nearly dry sticky droplets of paint adhering to the surface coat...

YMMV... OL JR

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Wayne's World said:

If the plastic has a waxy feel, it's probably polyethylene, which has the peculiar property that nothing will stick to it. I know that Sunward uses polyethylene in their canopied fighter cones. You have to sand with an agressive grit, and cross your fingers. If the parts are polystyrene, then there's a possiblilty that the Chinese use a funky mold release agent. If it's styrene, I recommend washing the parts with Dawn dish liquid, which is a decent grease cutter. You might try using Krylon Fusion, which is specially formulated for plastics.

More than likely, it's the crappy overseas mold release.

To tell the difference between styrene and polyethylene, dab the part with acetone or liquid model cement. If the solvent doesn't leave crazing on the part, then it's polyethylene. Nothing sticks to polyethylene.

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