Yes it has to be done outside...... we waterproofed old Scout tents with a mixture of parafyn wax disolved in naphta........ a similar mixture was sold at Canadian tire in the 50s......
A dangerous explosive mixture to make as you need heat to warm up/melt the parafin and you stir in the naphta......... we used a single burner electric counter top stove with a double boiler arrangement and a long extension cord to be away from the garage and house....... no smoking allowed. The tents were set up and we painted them inside and out with large 5 inch paint brushes..... doing the inside was fun as you would get dizzy and headachy all at once.... no beer needed.
With the right mixture the canvas would dry fast...and be very water tight.. and remarkably not sticky....... smelled for at least the first two camping trips......
Ah the good old days when nothing was toxic....wonder we are still alive!!!!
BooBee
PS...... I have about 100 pounds of parafyn blocks in the barn....just need the naptha.....and a few volunteers.......hihihi