Tried to start one of these threads a long time ago and no one seemed to believe me about the quality of this glue and its purpose in the aquarium industry. I am more than glad to know people are now catching on and following my advice and practices of using #40. I know some people are probably tired of hearing about it but I've been getting a lot of p.m.'s about this product and how to use it. #40/#42 is the only glue anyone should be using to repair an acrylic tank, it is 30 times stronger than weld on #4 and 60 times stronger than #16 (this information is provided by the weld-on company themselves not me). It is a two part epoxy that is simply mixed and applied. #42 is #40 being sold in the smallest "kit" they make, it pre mixes for you like when you buy an epoxy from home depot in the plastic tubes. If you have an acrylic tank chances are it was built with #40 in the first place. A lot of people build with #4 or #16 and it is good for small applications like when using 1/4",3/8",1/2" if your good lol.... I've built many acrylic tanks with #4 and #16 for years but still reinforce the inside of them with #40 to prevent any crazing or seam popping down the road. Tanks 15-20yrs old came "stock" with #40 tip and poured on the inside of them and I really don't understand where this practice was lost on tanks being sold today. My opinion is newer tanks are being made cheaper and newer builders just don't know about the product or are making more profit by using cheaper acrylic cements. #40 first and foremost can be used to weld together any thickness of acrylic with crystal clear seams. Its second purpose is using it for inner reinforcement that looks similar to silicone on the inside of a glass tank. #40 can prevent and repair any seam "crazing" or separation. Any used acrylic tank I buy, I take the time to reinforce them with #40 just to never worry about them lasting for my lifetime. I have tanks over 20yrs old that were made this way and there still going stong today. That's enough backstory I guess lol, anyone with questions about the product or how to use it feel free to ask here. Also feel free to post pics of the work/repairs you've done with #40 to help out the rest of our community. Boring read I know, but it will help a lot of us. i'll get some pics together of tanks ive done with it and the "tip and pour" method of reinforcement. Some people may think reinforcing tanks is "overkill" but it sure helps me sleep sound at night and I honestly prefer the look of it on an acrylic tank. #40/#42 can also be used to "butt joint" two pieces of acrylic on end next to eachother to make longer lengths of acrylic. That's how strong it is. #40 is comprised of the same chemical properties of the acrylic itself. It is more or less liquid acrylic. Just the same as what they mix when casting sheets of it.