How much primer?

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PostPosted: Tue Jul 13, 2021 12:49 pm
Hello I have a question about painting. I’m not pro by any means I’m not even an amateur. I use rattle cans a lot because it’s convenient. I always question myself wether I am putting on the primer thick enough. When I take the paint off a part that was professionally done the coats are thick. The cans say 3 coats, but after the 3 it doesn’t look right. For example now I’m trying to paint a crank case cover. It has the lawyers Yamaha there. I filled it with bondo. I sanded it all and added primer. I’m over a half a can in to it and I can still see the letters plain as day. Won’t this bleed through the paint? How much primer does it take in reality?

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PostPosted: Tue Jul 13, 2021 10:23 pm
Multiple problems here.

First, spray can primer is low on solids (so that it can be sprayed) and is pretty useless as a filler.

Second, and I suspect the major cause, is that your substrate isn't flat. Filler is softer than the aluminium surrounding of the lettering and it's very easy to sand it just that little bit lower, leaving the outline, as you have.

Third is the age old mistake of relying on primer to fix the faults in the substrate that should have been corrected before any paint is applied. The golden rule with painting is to keep it as thin as possible, thicker is not better.

Sand it all with P80-P120 then apply a thin coat of filler over the whole area, including the aluminium around the letters. Using a hard, not soft or flexible, block with P150-P180, sand it back until the covering over the aluminium is still there but almost see through. Lightly finesse the coarser scratching out with P240-P320 and then prime 3 light to medium coats. When fully dry, might take a few days, sand again with P600 dry or P800 wet, just enough to take any orange peel out. Then topcoats.
Chris

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PostPosted: Wed Jul 14, 2021 7:06 am
You're seeing the letters because rattle can primer is lacquer and lacquer primer
shrinks, it shrinks a lot for days. If you get enough on there and wait long enough for it to stop shrinking you can block it smooth and it will remain that way.
BUT, the more primer you add, the longer it keeps shrinking.
It could take a couple of weeks.
This is the main reason painters don't use lacquer primer anymore.
And, it is not for bare metal, it soaks up water like a sponge.
If you wet sand it after putting it over bare metal, it'll rust underneath.
JC.

(It's not custom painting-it's custom sanding)

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PostPosted: Thu Jul 15, 2021 8:28 am
JCCLARK wrote:This is the main reason painters don't use lacquer primer anymore.


I do. But the substrate has to be near perfect, otherwise as you say, you have to bucket it on and it then takes weeks to dry.... and shrink. If my repair is good I can use 2 light coats and wet sand in 15-20 minutes. But it has to be good.

JCCLARK wrote:And, it is not for bare metal,


I have a 4 litre tin of acrylic etch primer. Mix 1:1 with a nice fast acrylic thinner, spray as above and instant rust protection. As good as epoxy? No, but fine for smaller areas and it does actually work. Also see following comment.

JCCLARK wrote:You're seeing the letters because rattle can primer is lacquer and lacquer primer shrinks,


Which is why I suggested that he sand back the filler until it is "almost see through". Thus a layer of the filler remains on the alloy and it will stick better than the spray can primer but, more importantly, by not going all the way down to the aluminium you're not making a hard edge that you sand off and create a step, thus outlining the letters. I didn't mention it but using a guide coat when sanding the filler is a given.
Chris

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