I use Krylon workable fixatif spray coating no. 1306 to coat over pencil drawing on bare metal and transfers. Spray lightly. It does help to keep your design from rubbing off accidentally. You can engrave through it and remove with acetone.
I just spray over the transfer with a couple of coats of satin sheen clear lacquar to protect the transfer from damage. Stays on and you can cut through it.
I've been using the cheapest hairspray (Aquanet) over "Transfer Magic" transfers and it works fine. It's an old scrimshanders trick for fixing pencil lines on ivory, but works great on metal also. It cleans off easily with alcohol or acetone. Just use the lightest of coatings. The cheaper hairsprays are the best for this application since they are nothing more than a thinned laquer.
I use hairspray as well. I think the can cost me about $1.20 and will last a very long time. Just a light coating keeps the transfer from being rubbed off while cutting without flaking off and causing other problems. I do alot of lettering, calibers and company names on custom rifle barrels, and it can be tough not to rub some of the transfer off. The hairspray keeps it crisp and clean even on those really long shop names. David