I have the head of a 10 pound sledge hammer for my hand held anvil which works quite well.
Two ordinary carpenter hammers will not perform well..... you need a really SOLID backing anvil.
The rivets I used are 1/4" diameter by 5/16" long. Pretty hefty. It takes from 4 to 7
whacks to get it flattened out like I wanted.
I tried putting the 5200 on the rivet first - - - theoretically, it sounds like a good idea.
BUT - in actual performance, you are just wasting your expensive 5200 because after
half a dozen whacks of the hammer, there is no sealant left around the rivet.
The same with Pop Rivets - I noticed no sealing significance of applying the sealant before popping.
So, I installed the Pop Rivets and washers first, then a coat of primer, then I will seal with 5200
and come back with a second coat of primer. I will do this both inside and outside of the boat.
But, do whatever works for you !!!!