My fence was painted in the fall with water based stain. It needs a second coat. Can I now use oil based stain?
Yes you can but it is best to remove the old stain first. A power washer and injectable stain stripper will work well. The power washer can be rented and a good stain stripper purchased from your local paint store.
If you decide to not wash and strip your fence you could have peeling stain in areas where it doesn't soak into the wood.