Tuesday, September 21, 2010

Guide Coat and Beginning Block Sanding

Now comes what will probably be the single most time consuming part of working on this car: Block Sanding. In theory, using long blocks of stiff material to back the sandpaper, I must sand the whole car with a low grit paper (I'm starting with 180 grit), thus leveling all of the high and low spots perfectly. Then the whole car again with 400 grit, and then again with 600 or 800 grit. My next coat, the Primer Sealer, calls for the primer to be sanded to 400 or 600 grit. Check your product to see what grit you should finish with. I think 180 was plenty good to start with (no need to start block sanding with a 60 or 100 grit.), and the step from 180 to 400 was fine, with no between grits necessary.

You could do all this leveling by feel, or buy a spray-can of SEM 38203 Guide Coat (black). I've heard of people using old black paint (which can gum up sandpaper excessively), or using brush on black dust (which is more messy but sand off more easily). I got this because it was there and cheap: $6.

I decided to give a shout out to the forum that has been one of the best 944 resources for me, and to try my hand at graffiti, which I've always been too chicken to try. Spray painting words on a car felt all kinds of wrong... and fun. Good thing it sands off fast.

Once I was done playing around, I went around the whole car with the spray can. In retrospect, I was too close to the car, and ended up getting a pretty thick dark coat. It doesn't really matter, except that I used most of the can of guide coat up, when I probably could have dusted the whole car with half the can and gotten the same results.

Once the guide coat is on, you slap your sandpaper on your blocks and start sanding. Again, I'm using the Durablock 6 block system I got on ebay, which is already figured into my material costs earlier in the project. I will ultimately use mostly the 16" block for huge spaces (hood and doors), the 12" block for most of the rest of the car, with some limited use of the tubular block on the fenders and back of the car, and the short 4" block when the 12" block was just to big to get into tight spaces. If you are on a budget, you could do a 944 with just the 12", and 4", and pick up the round cylindrical block if you have a few extra $. The rest of the blocks weren't essential.

Below, you can see some typical block sanding in progress on the right side of the hood, looking from the fender (using a short block and 180 grit so you can see the different progress in one photo). On the far right of the picture, maybe the right-most 20%, you can see it is un-sanded. Just to the left of that, in the next 20%, you can see the "X" marks from me starting to cross-hatch as I sand a few strokes in each direction. (You always want to sand in an X pattern to keep from leaving grooves from the paper and to avoid the edges of your pad from digging in. There are loads of tutorials and videos on You-Tube about this.) Just to the right of the middle of the picture, you can see I'm taking the top off the rough texture, but there is still some substantial guide coat still left down in the bottom of the orange peel. This is also when you will start to notice high and low spots. The high spots get lighter, and the low spots stay dark. Keep sanding the whole panel evenly, but stop if you start to go into your etch primer, and stop before you go down to bare metal. On the left-hand side of the picture, you can see a few faint low spots that I was trying to sand down to, but there was one stubborn high-spot where I sanded down to the etch primer, which is clearly green. (Thank goodness for contrasting colors of primers. This would be a pain if they were both gray.) From here, you can do the wrong thing (avoid the high spot and just focus on the low spots, which leaves your panel uneven and with a high spot. Or you can hammer down the high spot. A great DIY trick I saw on a video was to take a thick solid metal spatula, hold it over the high spot, and gently hammer all around the high spot. The flat metal of the spatula distributes the force of the hammer, and keeps you from messing up the paint or metal. It also keeps you from hammering too far and ending up with a low spot. Spray on some more guide coat, and get back to sanding, hoping everything has evened out.

Here is some more work on the hood, with a couple of clear spots where the etch primer is showing through.

And a close up of the middle of the hood so you can get a better feel for what a 'sand-through" looks like. In retrospect, I shouldn't have gone this far, and should have fixed the high-spot a lot sooner. But I was scared to do too much hammering, hoping to get the panel flat before I actually got all the way down to bare metal. Don't do this, just hammer it out softly as soon as you see there is a high spot.

One other note. As you sand, the sandpaper loads up with the dust quickly, so that after a few strokes, the sandpaper is 100% covered in a layer of dust and won't sand anymore. You could take off your mask and blow the dust off, or use your hand to knock it off. I found that I could drag the sandpaper across the thigh of my jeans, and it would wipe the dust right off the first time, and I could get back to sanding. None of the books I read or videos I got addressed how to unload sandpaper, but this seemed to be OK.

Here I decided to do a time lapse with photos every 15 or so minutes showing progress on the hood, being more methodical from right to left:

In this next picture, you can see a microfiber rag. You can buy a pack of 10 of these from Costco for about $10. It is a great deal. I got one from my buddy. As you block sand, you end up with a panel covered in dust. If you blow it off with compressed air, or your lungs, you end up with a dust filled room. If you take your microfiber cloth and wipe off the dust, it gives does a better job, faster, with less or no dust in the air, and you don't have to take off your mask to blow the dust away.

Here, the hood is done, and many of the spots where I sanded down to etch primer also had some metal showing. I decided to be prudent, and used some more of the spray-can etch primer on those spots to keep the metal from being exposed. (After cleaning the dust off them and wiping them down with grease and wax remover.) The spray can etch primer is dark gray.

I was able to finish the hood of the car, including the headlights and valance between the lights, and the roof of the car (not including sunroof), in about 3 hours. Here is me being proud of my first big block sanding session. The first of many.

Time spent today: 3 hours
Money spent today: $6 for guide-coat, $0 for microfiber towel.

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