Ideally you need to apply it with an old gun (i use quite a big 1.6 fluid tip) and you can't let it stand in the gun too long as it becomes a nightmare to remove once it starts to set. Reface is also quite heat sensitive pre-activation, i suggest you bring it in the house and put it some where warm for a couple of days before use, if you activate it cold it will be VERY thick.
As for mask, i use one of these for Primer
https://www.ebay.co.uk/itm/3M-Medium-Du ... SwlR9cZard you might want to get some goggles too especially if you wear glasses.
One thing to take into account with Reface is it absorbs moisture so you need to flat it dry and seal it with a less porous primer over the top, don't have it sat for weeks in reface without sealing it because you will be asking for trouble....... Flat it dry and have a hoover on hand as it does produce quite fine dust that gets everywhere if not managed.
I haven't used it for a while but it doesn't tend to go along way, i try to put two nice wet coats on as anymore you start to loose sharp bodylines........ from memory to give you an idea how far it goes the last time i used it was on a Daytona Cobra replica body and that used about 1 1/2 liters (i didn't exactly bury it in the stuff). Obviously it's more like Body filler than paint so you don't have to let it flash between coats and it will still go off the same even if you pour it on (it does roll on quite well for smaller area's) as long as what ever you put it on is degreased and dust free it will stick really well.
Reface isn't a substitute for grinding out cracks and re-glassing (it is just a super high build primer) in a perfect world your repairs shouldn't need the extra build of reface to de-wobble, i tend to use it on cars i feel need the extra blocking.It's also handy if you want a barrier coat as it doesn't get attacked by solvents like allot of other high build primers which makes it very Cellulose friendly.