I have a 15' Aluminum rowboat with three small cracks in the hull. It has front and back decks, and I cannot fix them from inside the boat. I tried patching them using Aluminum puddy (it hardens to aluminum). The leaking has significantly slowed, but its still leaking. How can I patch the cracks from unde the boat?

 

Please dont direct me to tin boats *. I'm asking here.

 

Can I use flex seal???

I'd be looking for a welding shop that can do aluminum welding.

Cracks, like those you describe, may continue to "grow".  They are stress cracks caused by flexing.  If that is the case, the growth of the cracks can be stopped by drilling a small hole at the end of each crack.  Use a small drill, 1/16" or less.

 

I'd suggest using Gluvit.  You can paint it over the cracked areas once you have prepared the surface according to the instructions on the container.

 

http://www.marinetex.com/gluvit.html

I have had much experience with this condition in old aluminum boats. The only permanent solution is a newer boat. The next best thing is a competent welder (heli-arc for aluminum). After getting your present cracks welded you will still be susceptible to newer cracks forming. After aluminum gets older it starts to get brittle, hence, the cracks.

Drill holes like Rhino said, clean area well with wire brush or sand cloth, then use HTS2000 to fill, grind it til it looks purty

I fixed the crack in my old boat by the previously mentioned drilling, sanding and then applying JB Weld.

dont use flex seal i have tried it on my boat and it doesnt hold very well after it gets wet a few times it starts peeling off lol....i tried brazing rods but im not very good at it but i have managed to patch some of my cracks with a alum brazing rod and map gas  but i think it wont last long

 

another way if you dont weld it you can put a plate over it and screw it in and seal it with like 5200 or something...i might try that next lol...also there is some stuff from cabelas some stick but its sold out everytime i try to order it 

 

lol and i just looked and now it says the product is no longer available..........

When I had my old Tracker, I used this stuff call steel-flex.  the only place I know to buy it is in Hialeah, fl and the company hasn't made it out of the dark ages yet...  I can get the info if you want just PM me, the stuff worked really well for me, I had same gaps in the seam and it shored them up nicely. 

I've got my 12' jon at a TIG capable shop right now.I've had a few cracks in the floor from the boat bouncing and making contact with the rollers on the trailer.Normally the repairs take about an hour ($25 labor), but this time I'm having them reinforce an area where a previous repair was made, but a crack opened just above it.

The key is finding a shop that can weld aluminum (TIG) not all welding shops are equipped or capable.