Generally, the color is the indicator of the strength of the cured Loctite, but each color comes in several viscosites, from thin as water to thick as 90wt oil. Green (strongest), then red, then blue. The mil spec versions of the Loctites are the same as stuff we can buy but it uses letters (A for green, C for blue) and "V" after that for thickness/viscosity. Grade A is the thin, but strong, green (like 290), followed by AV, and AVV (like 680 cylindrical locking compound). Go by the color to judge the strength, and the viscosity based on the gap you are trying to fill. Also remember that a heat gun is often the preferred way to break loose a cured Loctite joint. So, beware of high temp applications. But Loctite does have lockers for high temp.
Primers are necessary when bonding non-reactive metals (stainless, titanium, etc) but not really needed for active metals (ones that corrode or rust easlily, like bare aluminum and steel.) But cleaning and lightly sanding to expose raw metal will improve the final strength. For example, Loctite bonds better to sanded aluminum than to anodized aluminum. Also, using a Primer will speed the cure of a joint very noticeably, no mater what metal is being bonded.
Personally, I don't get too concerned about the expiration dates, but I do periodically make some test pieces to verify strength. I have noticed it taking longer to cure as it gets older, but it is too expensive to just throw away. I'd be gone if that were the case, too! LOL.
