Should 4200 be applied to the transom before installing?

Assuming you are installing either a brand new assembly or at least an assembly with a new rubber gasket. If your transom is nice, clean and flat you shouldn't need any sealer.

I just put a new HD transom assy on a 20 year old boat. The transom was clean and flat. I installed it dry as well.

Same here, 20 some year old boat, installed dry, all good

My brand new transom assembly is going on a 26 year old boat getting a new transom in June. Feeling like I want to use some 4200 in there for good measure atleast around the thru hull studs whose holes can't be waterproofed.

when i redid my transom i sealed the exposed wood with fiberglass resin. that will waterproof the stud holes. i also did that to the key hole cut out

This sequence


These directions


Resin like mentioned, easy clean up

don't forget engine alignment.

Don't forget clearance for tiller arm.
supposed to be drilled angled out.

The foam seal for use on the transom assembly is all you need. There's a lot of squish built into it. Adding goop just makes it look like crap and unprofessional.
And don't forget to open up the top for the tiller arm as mentioned above.

That pic helps alot with figuring how to get resin into the stud holes. Thanks!