I agree with SJJohnston and JDowling that if you're a normal guy/non-racer, base repairs can actually be some of the easiest things to do yourself. You can pull it off no problem, allowing major savings over shop charges while giving you the least chance of inflicting serious and/or irreparable damage, compared to learning how to file and bevel your edges, et al.
Now look, man, don't go out of your way to prove me wrong, okay?! Because I'm sure you could dig right the %$# through those bases and find yourself coming out the other side if you really set your mind to it...
But here's a couple of tips on the epoxy method I use for filling deep gouges and core shots (and don't bother with p-tex candles--they're beyond useless):
TOOLS/MATERIALS: Use a sturdy utility knife, some 30-min 2-part liquid epoxy, and a sharp metal scraper (or an old blade from the utility knife) for leveling after the epoxy has cured.
The crux of this method is that the new epoxy material be KEYED IN to the existing base--so it doesn't fall out over time. This is easy--you just make sure when you're cutting out the gouged base material that you make angled cuts, making the area where your patch will go get wider the deeper it goes in the ski.
So even if the gouge doesn't extend to the core, I still do all of the following, to ensure a stable patch:
1) CUT AN OUTLINE AROUND THE GOUGE. Score a regular outline around the torn base material, then proceed to cut down through the base 'til you hit the layer underneath. Don't try and get there in one pass--take your time and work slowly down through the plastic.
2) REMOVE ALL THE OLD MATERIAL WITHIN THE PATCH AREA. Remove everything in the patch area down to the layer under the plastic.
3) MAKE SURE YOUR CUTS ARE BEVELED TO KEY-IN THE PATCH. As described above, angle those perimeter cuts so that the patch area is wider toward the core than at the surface. It's IMPORTANT!!
4) MIX THE EPOXY AND (OVER)FILL THE PATCH. Don't overmix it or you'll get bubbles entrained, which might then show up as gaps in the surface of your patch. And work somewhat quickly, because even though the work time is claimed as 30 minutes, the epoxy won't flow well after six or eight minutes, I've found. (Slightly) overfill the patch area.
5) LET THE EPOXY CURE. Yes, it might imply on the label that it'll set in 30 minutes, but the epoxy definitely needs to cure for 24 hours before you should attempt to scrape and level the patch.
6) SCRAPE THE PATCH UNTIL LEVEL. You can use the utility knife to carefully cut away really high spots, but it's usually safer to just slowly pull down the overfilled top of the patch, layer after layer, with a sharp scraper or old utility blade held at 30-to-45 degrees. This usually takes three-to-five minutes of repeated scraping per patch, so it's a little tedious, but it works well and gives a clean, flat blend with the surface of the existing base--if you don't rush it and just let the scraper do the work.
That's it. By all means, there's a lot of qualified folks on this forum who may know a better cheap technique--or may point out flaws in this one that I may be unaware of. But it's really cheap, really simple, and I've never had any problems with it in years of use; here's hoping the forum crew will sound off if there's anything to worry about.