From my experience it takes up to a year or more for a Sheltie to grow out of a bad grooming, never mind a complete shave.
Can you post pictures? Unless the dogs are really, really bad, I would focus on techniques to cut out the individual matts.
What grooming tools to you have? To avoid a full shave, you will need:
- Patience!
- Thinning Shears
- Straight Shears
- Slicker Brush
- Pin brush
- Detangling spray like Chris Christensen Ice on Ice or Isle of Dogs Silky Spray, for example
- Small round tip nail scissors
and more....if you are up to the challenge and want to avoid shaving, I and others on the forum can give your grooming tips. But I would avoid shaving if possible.
My two cents worth.