What it the 1st step to do when I want to apply oil base primer on top of semi-transparent stain + solid stain (both oil base)? Homeowner wanted to paint instead of stain. Started getting blistering in different locations of the house, when I applied Cover Stain, oil base primer. Its usually where the sun hits it. I think existing oil base stain was not completely dry, but it felt dry. I popped a bubble and it still feels sticky, but the outside of the stain felt dry.
A very good washing with a pressure washer and a mix of TSP and ammonia. Scrubbing the surface with this mix is also a good idea, use a small push broom or large scrub brush on an extension pole for the scrubbing. You could also use a commercially available injectable exterior wood siding stripper. The siding stripper is more environmentally friendly. A good washing removes some of the solvents and stain that is still present and opens up some of the wood pores.
Although it too late, paint isn't always the best choice. A really good solid stain, like Super Deck 9600 emulsified oil/acrylic, would have been a better choice.
In this case the prep is the same for a paint or stain.
Zinsser Cover Stain is a good primer but it dries to fast. The fast drying locks in some solvents and this reacts with the oil base stain causing degassing. Many oil based stains have Linseed oil, this is probably where the gases are coming from. The stickiness is the residue from the stain and solvent. A standard oil base wood primer with some Japan dryers would have been a better choice. Standard oil primer dries slower allowing some gasses to escape.
At this point your only option is to scrape all the bubbles and use a standard slow drying oil base primer.
This problem could happen again and again as the sun heats up the underlying stain. This will simply be a maintenance issue associated with using a paint over the existing stain.