Yep, I agree with Umms about very watered-down emulsion. If you start with a VERY thin coat, then you can easily build it up with more coats.
I've done this quite a lot on furniture ... you know, the old distressed look. Have a thought about "liming". It actually used to be lime, but modern emulsions work as well. You paint on, then rub it all down. The paint stays in the nooks and crannies. If too much is rubbed off, then you just put on another coat.
To pick up on Ratt's point. If you think it needs protection, then a coat of acrylic (water-based) lacquer will seal it all up. I've often used a simple acrylic floor sealer for this rather than polyurethane varnishes.
The craic is to practise on any old scraps of timber first.