What happens if you paint over dry rot?
Thereof, can you paint over water damaged wood?
Exposed wood soaks up water, raising the grain and creating a rough surface as the wood dries out. You can remedy this type of surface damage with some thorough sanding before repainting the bare wood. In more severe cases, exposed wood can remain wet for extended periods, ultimately leading to rot.
Then, how do you fix dry rot?
Repair wood with polyester filler: Apply wood hardener and filler. To repair rotted door frame, first remove rotted wood with a 5-in-1 or other sharp tool. Then coat the rotted door frame area with wood hardener as shown. Mix polyester wood filler or Bondo wood filler and press it into the recess with a putty knife.
You really can't "stop" dry rot - once the wood is rotted, that's it. You could alternatively add some metal braces to the wood that's intact. Other thoughts would be to "seal" the rotted parts with Kilz, primer and some oil paint.