What the alligator-skin pattern means
Sherwin-Williams' guide to paint failures describes alligatoring as patterned cracking in the paint film resembling the scales of an alligator. That's different from blistering, where bubbles form because the paint film has lost adhesion and lifted from the surface underneath. If what you're seeing on exterior trim is raised bubbles rather than a crack pattern, our guide to why paint blisters on exterior trim covers that problem.
Look closely without picking at it. Sherwin-Williams' alligatoring how-to notes that in typical cases the surface below the paint doesn't show through the cracks. Whether you can see bare wood at the bottom of the cracks matters, because it tells you how deep the failure goes.
Why old trim can undermine a fresh finish
The same Sherwin-Williams page lists four possible causes: oil-based paint losing elasticity as it ages through repeated expansion and contraction with temperature changes; a hard, rigid coating such as oil enamel applied over a softer, more flexible one; a topcoat applied before the primer or basecoat was dry; and a coating applied over a glossy finish. Note that Sherwin-Williams is a paint manufacturer and its page ends by recommending its own products. These are possible causes, not proof of which one applies to your trim.
On exterior woodwork, the National Park Service's Preservation Brief 10 on exterior paint problems on historic woodwork describes how the failure can progress. Cracking and alligatoring are advanced stages of crazing: once the bond between paint layers breaks, outdoor moisture gets into the cracks, the wood swells and the cracking deepens until it reaches bare wood. In advanced stages the paint also flakes badly. The brief is written for historic exterior woodwork, so don't assume every detail applies to interior trim.
Resolve lead concerns before disturbing the paint
Scraping and sanding are the usual preparation for alligatored paint, and on older trim that's exactly the step that can be dangerous. The EPA's lead-safe renovation guidance for do-it-yourselfers says homes built before 1978 may have lead-based paint, that the older the home the more likely it is, and that renovation, repair or painting work in a pre-1978 home with lead-based paint can easily create dangerous lead dust. EPA recommends hiring a certified lead inspector or lead risk assessor to test the surfaces you plan to work on, and says that if you can't determine your home's age, it's safest to assume the area has lead-based paint.
For contractors, EPA's Renovation, Repair and Painting program page says paint testing isn't required by the rule, but unless there's documentation that the paint isn't lead-based, the rule's requirements apply. If paint is tested before renovation, EPA lists who may do each type of testing: certified renovators with EPA-recognized test kits, certified inspectors or risk assessors with X-ray fluorescence instruments, and certified renovators, inspectors or risk assessors for paint chip sampling.
The repair depends on which layers are failing
Another topcoat isn't a substitute for figuring out what's failing underneath. The NPS brief says that if cracking and alligatoring are only in the top layers, the paint can probably be scraped and sanded to the next sound layer and repainted. If the cracking has reached bare wood and the paint has started to flake, it will need to be removed completely from that area. That's a judgment to make on the actual trim, after lead concerns are settled.
On exterior trim, the brief also stresses that moisture problems must be corrected before priming and repainting, because new paint will simply fail over a wet surface. Sherwin-Williams' how-to adds that each new coat should be given the drying time on its label before the next goes on, and that glossy surfaces should be sanded dull first. Ask for a plan that fits the actual wood and the coatings already on it, not just a different brand of paint.
Questions to ask before approving repainting
Ask the painter to explain which layers are failing, how much paint will be removed, and why the proposed primer and finish will work together. Ask that the written scope cover preparation, drying times between coats, and any moisture correction. If you still have old paint labels, application dates or photos of the trim over time, bring them to that conversation.
For a home built before 1978, also ask who will assess the paint for lead, whether the job falls under the RRP rule, and whether the firm is lead-safe certified. EPA says that, in general, anyone paid to do work that disturbs paint in pre-1978 housing needs certification, and firms can't advertise or perform covered work in those homes without it, subject to specific exceptions. Settle lead-safe preparation before work begins, not after dust has been created. Our checklist for hiring a home repair contractor in Philadelphia and Bucks County covers what to get in writing, and if your home is in Philadelphia, check whether historic designation rules apply to exterior work before repainting exterior trim.