A painted deck can be stained, but it will take more effort with the preparation. The original layer(s) of paint must be removed before any stain can be applied.
The types of techniques used to prepare your deck depend on the condition of the paint. I will outline each step I would use and let you decide if all of them apply to your particular situation.
1) Power Washing – A good washing with a pressure washer can remove some of the paint. The main advantage is the pressure washer will inject water under the existing paint film, helping to loosen it which will make its removal a little easier. Use about 2500-3000 psi and take your time. Try not to gouge the wood with the pressurized water.
2) Sanding – After the deck has dried all remaining loose paint must be removed. At this point you will have to decide what type of deck stain to use, solid-color or semi-transparent.
If a solid color oil based deck stain will be used just remove any loose paint and feather out these areas for a better look. For a semi-transparent or semi-solid deck stain all of the remaining paint must be removed. Any remaining paint will interfere with the deck stains ability to penetrate and the remaining paint will show through the stain.
Power sanding is the only way to remove the paint from a wood deck. There are several different types of sanders that could be used. These range from large 8 inch power grinders with sanding discs (the type I like to use) too rental equipment designed for sanding hardwood floors. You will have to do some experimenting with different equipment before you find the right combination.
3) Heavy Build Up between the Floor Boards – This part depends on the final look you are after and the type of deck stain used. Generally, any heavy or ugly buildup of paint should be scraped and removed. Use the sharp pointed end of a 5-way paint scraper.