>Can someone recommend a good spray to secure longevity of
>transfers on Acid Free watercolor paper?
Your question is not off-topic at all!
I assume by longevity you mean UV protection? There are some UV sprays on
the market that some folks use for Polaroid transfers. I've never used them
before. I recently got a Krylon one, for a series of transfers I just
started, but I haven't used the spray yet.
You might also ask your question on the image transfer mailing list (larger
audience for your topic).