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Spraying again do we still use wet n dry

PostPosted: Wed Apr 17, 2019 7:27 pm
by Concrete-crusher
Hi I am respraying a new colour, changing from carnival red to a more subtil wedge wood blue,

This time I have taken the old paint off back to gel coat, and am building back up to primer stage.

Miles Wilkins book states the primer should be left several weeks for solvent to escape, also it refers to using wet n dry before proceeding.

The cellulose primer data sheet however says to overcoat within 6 hours, I asked the paint supplier about this and was told all cellulose primers are porus so should not be left for long periods. That's fine but if true does using wet n dry create a new problem with moisture being introduced?

Or should wet n dry be avoided altogether or only used on the final colour coats

By the way my previous spraying around 4 years ago was just starting to show tiny micro blisters and I did wet n dry that

Steve

Re: Spraying again do we still use wet n dry

PostPosted: Wed Apr 17, 2019 9:00 pm
by nmauduit
From my limited (and not always lucky) experience, if you have a continuous film of sealer on the gelcoat or on the build primer above the gelcoat if applicable (assuming it was dry sanded) then you should be ok - I would still dry up (40° celsius for an hour) to make sure the solvents of the degreaser etc are not trapped below... I guess the way it would work for your cellullose system would be specific primer then x layers of paint, which you may wet sand in between or after a few layers, provided you dry the body before applying new layers.
What is risky is trapping water or solvent in porous areas way below (either exposed gelcoat cracks, exposed fiberglass, porous build primer, touch up filler etc) and covering that with a water/solvent tight layer (like paint).

good luck !

Re: Spraying again do we still use wet n dry

PostPosted: Wed Apr 17, 2019 9:55 pm
by Grizzly
Rub the primer with some dry 800's if it dusts it's dry if it clogs the paper it isn't.

IMHO i'd stick to what the data sheets say, there are too many different products out there to have a 'one answer fits all'

Personally i'd never leave any primer for 'weeks' even if it's air drying, i'm sure you could but there is an ideal time to flat primer before it goes so rock hard it takes you two or three times the time to do the job for no benefit. Cellulose primer is ok but just remember it is quite reactive to thinners and there are plenty of better options too use as a base even if your using Cellulose as a top coat.

btw the wet flatting thing is BS, the only primer i wouldn't wet flat is Reface (obviously don't wet flat bare grp or body filler) you should be panel wiping before any paint application and that will remove any surface moisture/oils.... if your applying primer out side a booth try to get a day with low humidity thats not too hot.

Re: Spraying again do we still use wet n dry

PostPosted: Sat Apr 20, 2019 9:00 am
by nigelrbfurness
In 45 years of refinishing in cellulose I've never wet flatted primer. Dry flatting with progressively finer wet and dry up to P600. Two fast colour coats then wet flat, dry off; tack off then solvent wipe then start to build up the colour, wet flatting as needed every couple of coats. Very early on in my career I saw the results of wet flatting primer on an Elan and the owner was beside himself as the car developed a rash on it's beautiful red paint job. There is no need to wet flat primer so why take the risk?

Re: Spraying again do we still use wet n dry

PostPosted: Sat Apr 20, 2019 10:43 am
by Grizzly
Thing is i can think of 5 more reasons moisture would be introduced into a paint job thats more likely than wet flatting if done properly.

I always wet flat primer simply because any moisture at all will cause the higher grade dry paper to clog and that will introduce marks into the primer, it's not so obvious with solid colours but cellulose metallic's are very susceptible to it as it does odd things with the metallic flakes. The biggest reason i wet flat is cleanliness.

In my professional experience all painters have their own ways of doing things, i know people that paint straight over flatted primer with just a quick tack off but thats not my style, i prefer to 100% make sure all moisture and oil's are removed with at least two passes of solvent panel wipe, also because of the types of cars i paint the compressor/dryer is kept in immaculate condition.

Just read the date sheets and do as they suggest, it's true there are hydroscopic primers out there but you tend to find it's only the cheap stuff...... Cellulose hasn't been the same since the lead was banned to be quite honest.