Sealey cavity wax gun

I assume you're talking about the type that works via the compressed air supply, if so, have you tried giving it a really good clean with thinners? Cavity wax can go semi solid and cause a blockage.

It seems happy enough shooting thinners through, but I'll give it a second go. Yes, I'm talking about the kind of thing that runs off a compressor. I didn't think there was another.

Also, is this a suction feed device or the pressure pot type wax gun? You might have a thicker mix of wax and as such try (carefully) heating the wax up a little - don't set fire to anything nor burn yourself with hot wax! :~(

Yes, Waxoyl (so other might also) do a version were the unit fits into the top of the wax can and has a manual pump like a garden weed killer spray.

Suction. I've tried heating and thinning it. I'm beginning to think it's a poor design as the pressure pot seems more common.