What is a good way to paint over paint on wood?

[Reutte]

So I'm following that lovely bit of advice about how to learn shading/lighting by painting wood blocks white and then pose them and draw. (Can't remember who shared that on this forum but I read it here a few times.)

Anyways, I've found a good block set that seems to work since the blocks are small, there are a good variety of shapes and it's relatively cheap. I never knew how expensive good quality kindergarten blocks were until I started looking.

http://www.amazon.com/gp/product/B00...pf_rd_i=507846

The problem is that the blocks are all painted, rather than natural unprimed wood. I will have to paint over them with white but I'm not sure if I need to strip off the colored paint off of the blocks first with sandpaper or something? Or should I just use spray paint and it will stick? I thought of using Tempera paint but I don't know if it will stick over the already painted/finished wood. Same with acrylic. And I'm pretty sure that house paint or wall paint isn't the right type to use either.

Most of the information I'm getting from google refers to housing or furniture. Not smooth wood blocks.

Thanks for any help.

[dpaint]

I would imagine a good white acrylic enamel spray should be just fine.

[Elwell]

Flat white spray primer. If they're super shiny it wouldn't hurt to give then a light go-over with fine sandpaper or steel wool to give them a little tooth, but don't worry about taking the paint off.

[Stoat]

Acrylic white gesso would be good, too. Sticks to anything and gives it a little tooth.

[Reutte]

Thanks everyone! Unfortunately, the store didn't have the blocks so I'll have to order them online. But once they get here, I'll check out these options and see which works.

[Cola73]

Before priming, it may be a good idea to check the condition of the blocks.
If there are cracks in the previous paint layer, it's possible there's some rot as well, so you could impregnate the whole block first in a precaution manner, let it dry thoroughly and then prime and paint it.