HI all, I live in Quebec Canada (God bless winter), I have a hairline foundation crack, I heard briefly on a radio talk show that I could use an "autobody fibreglass repair kit" to seal the crack from the outside once I dig it up. . .
Many small hair line cracks are repaired fine from just the inside or left well alone. You being up farther north then I am used to it might be somewhat different. In northern Illinois We sometimes get up to a 4' frost line. I would assume your is deeper. However even in that frost line area the out side of the foundations here seem to stay warm enough so the penetrating water doesn't freeze enogh to heave it. I would seriously make sure the solution you choose is WELL proven if you plan to dig the outside. There are many sealers that contractors can a apply and there is hydraulic patching cement made just for such repairs. I cannot confirm or exclude the possibility of auto body fiberglass. But I don't think I would be leaning toward it for my first choice after digging up the hole!
By the way a "small hair line crack " may not even bee seen on the outside wall to apply the filler!