Also be aware that over time, a coat of smooth-prime that was originally
sanded with 320P one day after spraying will start to smooth over the
scratch. In other words after a week your 320P scratch may now be 500P or
600P. It's still thru curing for a while. Point being, don't sand your parts
on Monday with 320P and paint on Saturday and hope to have good adhesion.
You may not. BTW I don't believe adding another coat of primer/sealer over
your smooth-prime just before topcoating has any effect on adhesion. Your
still putting a wet layer down with a mechanical bond only. If you are
trying to hide multiple colors of primer or something then a sealer coat is
wise, but not for adhesion. More wet coats can give you problems such as
dye-back and solvent entrapment.
James Thursby
-----Original Message-----
Behalf Of Fred Fillinger
Subject: Re: Primer and Paint
The difference on solvent paints, where wet-on-wet came up, is
possible interaction of the solvents in primer and topcoat, plus
locking in unflashed solvents by the top coat, but depending on how
fast the topcoat cures. Water-bornes are different, with no solvents,
as water can migrate via osmosis pre-cure. Don't they tell you not to
wet sand unless really fully cured?!
Everything you observed per below I found with System Three's
water-borne, too. Once cured, only tooth adhesion rules. Something
about how in the cross-linking process the molecules form chains for a
new molecular structure that don't want to adhere to its own kind on
subsequent coats.
Best,
Fred F.
"Tony S. Krzyzewski" wrote:
>
> Re "wet-on-wet," the topcoat mfr's directions should control.
>
> I found that without applying wet on wet with Top Gloss I got inter
> layer disbonding. My standard spray technique now is to mist on a light
> coat, leave for 20 minutes and then spray another two coats at 30 minute
> intervals applying as much paint as possible without getting drips (now
> there's an art!).
>
> Absolutely essential with Top Gloss is a good tooth grip - nothing finer
> than 320 grit. Miss a spot with the sanding or fail to clean it properly
> with Prep Wipe and there's a good chance it'll break and peel back ....
> just like the front area of the lower cowling that I painted last
> weekend :-( If you have to respray the stuff then it's out with the 320
> grit sandpaper before you spray.
>
> Another advantage of effectively building up one thicker coat is that
> there is no opportunity to sand through a coat to expose the lower one.
> If you do build up a multilayer finish and cut through the top layer
> while finish sanding you'll find that there's a tidal mark which is
> impossible to remove without painting again.
>
> I would also add that if you get it right then Top Gloss is very tough
> and polishes back beautifully.
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