Waterproof a canvas bench swing?

Canvac is the best canvas preservative, you can get at you local canvas shop but I think you will waste your money, Canvac is expensive, when you said there was mold and moss on it it's most likely shot already, when it drys real good and is in the sun is when you will see it disintegrate if it's to far gone but you may have caught it in time, it most likely was sewen with cotton thread which will rot with mold, the best thing to do is to spray it with inexpensive fabric protector, valspar is one, there a lot of them. Be easy with it and it will last awhile, having a replacement custom made will be more than the swing cost new.. Good Luck ..

Hosing the wood won't hurt it if it can dry out fairly fast. Sanding would be the best,but is a long and tedious job. Being outside,you can try,Easy Off Oven Cleaner ( I only use the one in the Blue Can,Fume Free) it should remove some of the dirt,and it will remove the paint,so be care full, You can also wire brush it. You should repaint,or use a Clear Coat. You can also use a Waterproofing,but that usually takes 45 days to cure. If you do repaint,use only a Exterior High Gloss. They are the most durable of the paints.

There are spray or brush on products for this. They would be sold in camping/outdoor stores or departments. The brand sold in my local stores is "Escort Canvas Waterproofing" but I'm in Canada so I don't know if that brand is available in the US. If not, there'll be something similar.

Visit and art and craft store and ask if they have any canvas sealer or anything of the sort.

I would also recommend asking the people in the store if they could help at all. Also try a fabric or upholstery store.