wrinkly varnish

[wood spirit]

A while ago finishing a tabletop using up old polyurethane small patches would wrinkle up. What was peculiar was that the first coat went on fine, but it happened on the second coat fairly quickly, repeatedly in the same spots.
Finished it eventually with sheer bloody mindedness,but have never come across an answer to its cause.
By the way 1st coat was left to dry for 3 days in warmish weather-so I don't think that's the problem.

[Big Shed]

Wrinkling of surface coatings, especially on the second and subsequent coats, is invariably due to to the preceding coat not being properly dry and the solvents in the subsequent coat attack this partly dry previous coat.

This lack of drying is usually caused by a phenomenon called drier absorption. As paint gets older the driers in the formulation lose their effectiveness, usually more pronounced in pigmented coatings, but it can occur in clear coatings as well, especially as the drier/anti-oxidant balance is incorrect.

If there was a thick skin on the surface of the varnish in the can, this is where some of the driers disappeared to.

Polyurethane vanishes dry by the driers reacting with air and cross linking the resin, if there are insufficient driers left the resin in the coating will eventually air-dry but it can take a long time, certainly longer than 3 days.

[Bob38S]

Same place etc each time. As has been said re old (probably beyond shelf life due to age, humidity, heat etc in the shed) finishes.

However, I do have another suspicion, the dreaded silicone - fish eyes in the finish or if the finish is thick or old you could have wrinkles.

The reason for my suspicion is that it occurs in the same places each time. You would be surprised at how easily the contamination can occur as well as how difficult to near impossible it can be to remove.

Used any sprays upwind? Any rags with sealant residue or solvent used on fresh caulking? Is it possible that the contamination occurred before you got the material or during its transport to your shed?

Just a thought.

Perhaps the only way to find out would be to thoroughly strip the old stuff and try some new.

[Big Shed]

Yes Bob, contamination is another likely factor, but in that case it is more likely due an oily substrate (either the type of timber or external contamination), not so likely to be silicone if it wrinkled.
As you said, silicone is nasty stuff with any surface coating, but usually causes fish eyes, not wrinkling.