Paint doesn't adhere to the 320 sanded particleboard surface. Oversanded?

Hi,
I made a "laboratory test" for painting particleboard. I sanded the particleboard up to 320. Then applied sealer. Sanded again with 320. The surface feels very soft. Then I brush painted it with alkydalic anticorrosive paint with a brush. After some days, when I pass the fingernail hard over the painted surface I can give off the paint up to the original surface. It looks like the paint doesn't "stick" to the surface. Then I read the small letter instructions on the paint can (I agree, I should be read it before but...). It says the surface should feel a little harsh before painting. So it looks I oversanded. The problem is I sealed and sanded all my "production" doors already. Now I should back sand all them but up to which grit? Which maximum grit do you recommend?
Thanks in advance
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Faustino Dina
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how about some primer?

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Brazil one. So it means possibly should I use a primer over the sealer? Actually the paint can (another local brand) recommended a primer (of the same brand, of course) but I thought primer and sealer was the same thing and I've used one yet...
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you can try. I know paint doesnt stick to alot of surfaces without a primer underneath to give it tooth. for example car paint.

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my "classic" problem is now to locate who sells the primer advised by the can. I couldn't locate yet a full line of finishing products in my locality... Thanks a lot for your help
Faustino
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Faustino Dina asks:

For painted surfaces, I don't go beyond a 120 grit. If you want to go hog wild, go for 150.
Charlie Self
"In the final choice a soldier's pack is not so heavy as a prisoner's chains." Dwight D. Eisenhower
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I used a 1/2 lb. cut of super blond shellac as a sealer and intermediary to paint in a similar project. Practically everything sticks to shellac.
Wood-to-Dust Guy
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