Re: Cracks in alluminum hull
Personally (and in the face of many opinions), I would, as has been noted already, clean the crack area well, but before "roughing", I'd drill a small hole in each end of each crack (stop drill).<br /><br />If it cracked, then it underwent stress at the crack point and you likely don't know when or how that stress occurred. If further cracking is not prevented, any existing crack will lengthen should the same stress be encountered again.<br /><br />By small, I mean 1/8 to 3/16". The hole has to be of large enough radius to distribute any future stress loads.<br /><br />Now, score along the crack lightly with some pointed object like the scribe in your tool box or the back of a hunting/ppocket knife ... not as thin bladed as a kitchen knife. This cleans and slightly bevels the crack edges.<br /> <br />Then roughen the surrounding area to give the aluminum some "tooth" for the epoxy to grab onto. Don't get carried away here or you'll just thin the surrounding aluminum thickness.<br /><br />Finally, clean the area with denatured alcohol (leaves no residue, but any aromatic will do, like acetone) and apply any good quality slow or medium cure epoxy (30 minute working time) that has been thickened to paste consistency with either Aerosil, Cabosil or very fine aluminum particles. With a fine file and a chunk of scrap aluminum, you can generate 1/2 teaspoon of aluminum powder in about 2 minutes. It should be about the consistency of Peanut Butter. <br /><br />If you get it too thick, epoxy can be thinned with a few drops of Mineral Spirits.<br /><br />The best application tool I've found is a 1/4" wide wooden coffee stirrer. A popcicle stick is too stiff and too short for my taste, but can be used, particularly if you thin it a bit with some sand paper. Or, you can cut your own off a 2X4 edge with a good knife - just make sure to trim all the "fuzzies" off to get good smooth edges/surfaces.<br /><br />Apply a thin coat of epoxy over the crack, INTO the "stop drill" holes and to about 1/2 inch around the crack feathering it out at the edges to blend with the hull. A layer about 1/16 to 1/8" think is plenty (think of putting a dime over the crack, but no thicker than a nickel. <br /><br />Rig something that will allow you to heat the crack area to about 140 to 160 F. Often just a cardboard box over the area with a 75 watt light bulb inside will generate that much heat. I've also rigged a three sided cardboard box over epoxy cures and blown in hot air with a small ceramic heater set on medium. Put a thermometer inside and close to the crack to monitor the temps so you can sdjust the heat source spacing. Too hot (over 180F is not good). <br /><br />The hotter (up to a limit, of course) you cure epoxy, the harder it cures and the tougher it becomes to future failure. The method I've described yields a bond with a shear strength of over 3000 pounds and an impact fracture strength of 4000 pounds.<br /><br />Cure it for at least 6 hours at high temp (any more than 8 is a waste of electricity) and then let it cool inside the box to ambient temperature for another 8 hours. Over night is good.
<br /><br />Check by phone with some of the epoxy distributers or manufacturers you can find online. Most are very helpful and some I've used are:<br /><br />John Greer (jgreer.com) 877-342-8860<br />Mas Epoxies (masepoxies.com) 888-627-3789<br />Us Composites (shopmaninc.com) 561-588-1001<br />But there are loads of epoxy resources on the Internet.<br /><br />While I do like JB Weld for many applications and while it may serve the purpose here just fine, what I've given you is how I would do it. <br /><br />Good luck.
