Proper tyre weld poses no problem to tyre repairs. Its solluble and just hoses of and is no impediment to an effective repair. The National Tyre Distribution Association endorse it.

Slime is OK, but doesn't inflate the tyre like a tyre weld product does. Put it in a healthy tyre and it holds off punctures, but won't seal them for ever so you're tyre will still go down, just at another time.