RE: 5 minute epoxy vs. 30 minute epoxy?
I'd agree with Jaka that in our applications, there isn't a significant diff in bond strength between epoxies and CAs. I like and use both, but epoxy is still my overall fave adhesive because of how versatile it is (that and I mainly build composite airframes.)
Technically, most 30 minute epoxies are a bit stronger than 5's due to the longer cure time, but I believe it is only like 10% or something in that range. Heat is another factor in cure strength as most epoxies will cure harder if cured under heat, generally in the 150F range, but it does vary and a quick blast from a heat gun won't do anything. The epoxy has to be baked for hours at this temp and the heat must be applied during the gel stage all thru till final cure.
The main diff between 5 and 30 is work time as pointed out in previous posts. It is difficult to set parts with 5 minute epoxy and gain good penetration, which can result in a weak bond. But this isn't the fault of the epoxyâ€™s strength. The epoxy is fine, it is the bond that fails. You can add heat to lower the viscosity which will greatly increase penetration, but it also greatly decreases working and cure time and if too much heat is applied, the epoxy can blister, which will weaken the joint. One of the best ways to ensure good penetration and hence a good bond is surface preparation, i.e. sand and clean.
When all is considered, I wouldn't be afraid to trust my planes to all 5 minute epoxy as the strength is more than sufficient. However, in practice this isn't feasible as the 5 minute stuff is difficult to work with because of the rapid cure times. For that reason, I only use 5 minute for small items that I can align quickly. I also find 30 minute too fast sometimes, especially if I added fillers to the epoxy which tend to cause it to kick quickly. For example, 30 minute epoxy at room temp with the right fillers will kick in about 5 minutes. In those situation I often use a very slow cure mix, like a 2 hour formula.