Sanding paint - what grit to remove brush marks

[Estoril-5]

As the title says really.

I apply regular household paint to MDF once it's sealed, I apply 3 to 4 coats but each coat leaves brush marks (I don't use a roller or have access to a spray set up).

After the last coat has dried I used p320 wet and dry (used it dry) and it just cut straight through the paint.

What grit should I be using to leave a smooth finish?

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[thetyreman]

maybe try a sanding sealer first, then paint over it, that should improve things.

 

[Estoril-5]

I put on a sealer before applying paint.

How would a sanding sealer help?

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[ED65]

This is going to look like a faff, and honestly it is a bit, but if you insist on doing it by brush do all of these:
1. Learn to feather off.
2. Slightly thin the paint. Don't thin in the original container; decant and dilute.
3. Use a brush with finer bristles. What works so well for enamel or oil paint (a good hog bristle) sucks for water-bourne paints. Synthetics are the way to go for effective and affordable brushes.
4. Dampen brush before you start (good advice with any finish, especially helpful with water paints).
5. Don't overload the brush, dip no more than 1/3 of the bristle length.
6. Unload the brush against a straight edge, not a curved one.

I don't use a roller...

Any particular reason why not? You can try the above tips or use rollers 8)

IME a roller beats any number of tips on getting a smooth surface by brush, because it takes no great effort – and basically no practice or experience – to get a better surface. A roller that will give a great surface is also cheaper than any half-decent brush. Their only major downside IMO is that they're such a hassle to clean out but I think it's worth it.

If you just have it in your head that rollers are the work of the devil then I'd recommend you try paint pads.

About the paint you're using, these can vary quite a bit surprisingly once you get down to it but there's a good chance you didn't wait long enough before sanding. Because it's a water-bourne paint we tend to think that once it's dry it's dry, but these paints too need to cure. They can be much tougher after two or three days or a week than after just a few hours of drying, even in the height of summer.

That said these paints tend not to sand well anyway because they're not abundantly pigmented and have a very flexible binder.

 

[Phil Pascoe]

^^^^ many rollers aren't worth cleaning out. For small jobs, use a radiator roller. Just wrap them in clingfilm, they'll be fine for a day or two and when they get dog eared throw them away. ( If using oil based paint the roller is often cheaper than the white spirit it would take, anyway).