Look at the conditions when the paint went on
Sherwin-Williams' guide to paint blistering describes heat blistering from painting in direct sunlight on a surface that is too warm. It also lists oil-based or alkyd paint applied over a damp or wet surface, and latex paint exposed to dew, high humidity or rain shortly after it dried. Insufficient surface preparation, or painting over a contaminant the coating can't stick to, can cause blistering too.
Before you talk to a painter, write down what you know: the product used, when the trim was painted, how it was prepared, whether it was in full sun, and whether it rained or turned damp soon afterward. Ask which explanation that history supports and what is still unknown. Our guide to the photos to take before a repair estimate can help you record where the blisters are and how they're spread out.
Consider moisture reaching the trim
The Pacific Northwest National Laboratory's Building America Solution Center, in its guide to missing window and door flashing, says bubbling, blistering and peeling paint can be a sign of moisture damage underneath: when damp wood dries in the sun, the escaping water vapor can lift the paint. It names several ways water reaches trim around windows and doors, including missing flashing, a poorly draining window, splash-back from rain where there are no gutters overhead, and snow that keeps building up against the trim. It also notes that door trim whose cut ends were never primed or painted can pull water up through the end grain if those ends touch the ground.
The same guide points out that without removing the trim or cutting into the siding, it's hard to see whether flashing was installed. So note where the blisters are, for example above a window, under a sill or at the base of a door frame, and leave the trim in place for whoever inspects it.
Where the blister separates tells you something
Sherwin-Williams says that if blisters go all the way down to the bare surface, the cause is likely moisture escaping from it. Where blisters go down only to an earlier coat of paint, its recommended fix focuses on removing the loose paint and sanding glossy surfaces dull before repainting. Its exterior troubleshooting FAQ adds that paint applied over old, hard, glossy coats, or over dirt and contaminants that weren't cleaned off, can fail to bond to the coat below. Ask whoever inspects the trim which of these they found and what they saw that supports it.
Fix the water, not just the paint
Sherwin-Williams says to find and repair any cause of excess moisture before repainting, and the PNNL guide says to work with a contractor to find and fix the source of a leak rather than only repairing the damage. If water is suspected, ask whether the plan covers where it is coming from as well as the finish, and consider a contractor experienced with exterior flashing and drainage.
More caulk isn't always the answer. PNNL warns against sealing the weep holes at the bottom of windows, because they're designed to let moisture out. Ask for a repair plan that separates the gaps that should be sealed from the drainage openings that must stay open. Our checklist for hiring a home repair contractor in Philadelphia and Bucks County covers other questions to ask before you sign.
Check for lead before anyone scrapes
Sherwin-Williams' fix for blistering starts with scraping off loose paint and sanding, and that is where older homes need care. The EPA's Lead Renovation, Repair and Painting Program page says any renovation, repair or painting project in a pre-1978 home that has lead-based paint can easily create dangerous lead dust. Its federal rule requires anyone paid to do work that disturbs painted surfaces in homes built before 1978 to be certified and trained in lead-safe work practices, and the EPA recommends that homeowners planning such a project hire a lead-safe certified contractor. Ask how lead will be handled before any paint is disturbed.
If you're in Philadelphia and the trim work is part of a larger exterior project, our guide on checking historic designation rules before exterior work explains how to find out whether those rules apply to your home first.
Sherwin-Williams is named here only because its public troubleshooting guides are useful references. HomeFix isn't affiliated with Sherwin-Williams and isn't recommending its products.