First, it depends on how deep the scratches are for how rough or fine of sand paper you use. Once you get to about 1000 grit sand paper you're okay to paint. Just for kicks, before you go sanding it, make sure the scratches have actually gone through the paint and not just the clear coat. If your clearcoat is scuffed it can look like a worse scratch than it is. How low in the bumber is it? The thing about sanding and repainting is going to be that it's gonna come out looking less than perfect for a DIY project unless you've got an actual spray gun, the perfect Subie color, and clear coat/knowledge of how to spray it. I've painted a couple of my own cars before in an auto body class in college, and I've also done my share of rattle-can jobs. Silver is one of the hardest colors to get to match, because there are so many levels of metallic fleck that manufacturers put in the mixture. It's also harder to find a color to match out of a spray can. The thing is that generally, you'll sand the entire area you need to paint, and even scuff it with a scouring pad all the way until the body line. You paint the are of paint you need to with the color, and then you re-clear coat the entire section, body line to body line. Then you still don't know if it will match. If it's on the bottom side that you have the scrapes, really, then go for it and don't worry about doing a clear coat. If you can see it from the top looking down at it, I'd personally deal with the scrapes until I can get it to a body shop to fix it. It'll end up looking really obvious that it's been worked otherwise. Now, IF the scratches aren't WAY deep, or you just want to see how you can get it before tackling the larger project, you can take 1000 grit sand paper and scuff it until the scratches are basically smooth, and then you can make it all shiny again of there's any paint left to shine. You'll need to use a sanding block when you sand it, otherwise you'll push harder in some places with your fingers than other, giving it a noticeable streaky set of scratches that will be a pain to buff out. Go to a local shop that sells automotive paint. Ask them for a rubbing compound and then swirl remover, then just some carnauba wax. They'll tell you you'll need around 4-5 total products, but these 3 are more than enough to complete the job. If you want to make the work really easy, you can go to a tool rental place and get a grinder with the velcro pad attachment. You can buy some polishing pads to work on the compound, swirl remover, and wax at the paint shop. This will save you a load of elbow grease depending on how much there is to it. Be careful not to burn the polishes into the paint though. Practice on something else to get the smooth motion and the pull the grinder will give you. Hold it at an angle or you won't be able to control the way it polishes. After the sandpaper, use the rubbing compound first, and do it just like wax. Rub it in like crazy, and leave a coat it to dry, then buff it with a polishing cloth. it'll no longer look all scratched, but if you see a reflection, then you'll notice the swirls. . . that's what the swirl remover is for. Same process, and then the same thing for the wax to give it a high polish and repel water, dirt, and whatever you come in contact with. PM me if you have any specific questions you may have about it and I can try to walk you through any of it. Sorry to hear about it, but you'll get some satisfaction from making it look good again. If you do sanding don't get anything much lower than 200 grit, as it will cut deep grooves into the paint that will be very difficult to sand, buff, and polish out. That's my .02 for you.