I've built several 6mm plywood boats with tape on the inside and 6 oz. fiberglass on the outside. Those boats have been adequately strong and durable. I've built several 3mm boats with 6 oz. fiberglass inside and out. Those boats have been adequately strong and durable. If I need more strength and durability on the 3mm boats I'd add more cloth at strategic locations inside and out on the hull. If I was using 4 oz. cloth on the 3mm boats I would use one layer inside and out. For more durability and strength I'd add another layer of 4 oz. cloth strategically placed inside and out. If I need more strength than that I'd have used 6 oz. cloth. I'm not even considering exotic cloths right now. I've never glassed a 4mm plywood boat only on the outside...but if I did...I'd be careful with it. There are those who advocate multiple layers of thinner cloth as apposed to one layer of 6 oz. cloth. For the average builder...I don't. There are always trade offs in weight and strength. Decide what you need...or want...and do it. Then...tell us how it works. I might add that my favorite lay up on an S&G boat is with 3mm plywood, 6 oz. cloth inside and out, with an added layer inside and out on the bottom of the hull. I also add whatever is necessary to strengthen the deck as needed.