Harley Paintin', the Missing Prep Article

By Dave Murray


When writing the "Paintin'" article, I wanted to keep it to a manageable length, so I did not cover paint prep. I have received enough requests for this information to warrant an article on the topic. Prep isn't hard, but it requires care and a willingness to get it right. Paint covers NOTHING! In fact, it forms a mirror, and will accentuate any flaw beneath.

Once again, we are discussing basic techniques for the amateur, to work with the accompanying article on painting with lacquer, using very basic equipment. I'm not going to cover electrostatically applied Hydrocortisone/Vanadium body fillers. There are many variations on the techniques below; you will find what works for you.

A word. About two dozen people have tried those painting techniques with good results. See Rabbi Moshe's article, and his stunning FL. I have, however, received two or three emails saying, "I used different materials, in a different order, and skipped three steps, and it didn't work out too well." Go figure.

Why prep at all? Steel tends to be rough, and paint doesn't stick to it very well. We need an intermediate layer that sticks to steel, which can be made smooth, and which will accept the paint. The same steps work with fiberglass and ABS plastic.

We will use three basic products:

  • Polyester Body Filler, universally known as "Bondo," a trade name. For filling dents up to about 1/4" deep. Deeper dents must be hammered out.
  • Glazing/Spot Putty, for filling very small scratches, pinholes in Bondo, and the like.
  • Sandable Lacquer Spray Primer, for the last pass before painting. It will fill light sanding marks, and little else. Comes in a spray can.

New Parts

If you have new, bright steel or plastic parts, and they have no ripples or dents, and very fine sanding marks, you can simply scuff-sand with "Scotchbrite" abrasive pads (red or green) or #400 sandpaper to remove the gloss. Degrease the surfaces with a commercial degreaser or Naphtha (lighter fluid), and shoot on two or three light coats of primer, 30 minutes between coats. Sand smooth with #600 wet/dry paper, and you're ready for paint. No steel should be showing through.

Imperfect Parts

The above scenario is as rare as Atheists in Foxholes. My new tank appeared to have been pounded out by a drunken blacksmith with a stone axe. Scuff-sand and degrease the part. Sand any dents and loose paint down to bare steel, but you usually don't have to completely strip the original paint, if it is reasonably hard, and is not attacked by the sprayable primer (check). Harley stock paint will stop a .357 Magnum, and usually doesn't have to be removed unless it's cracked.

Now, prepare for the "Bondo Experience." Bondo is damn near magical, when compared to the Hot Lead techniques that preceded it. It's a putty, about the consistency of peanut butter, which cures with the addition of a hardener (sometimes called a "Catalyst"), that comes in a separate tube.

Bondo is usually gray, the hardener red or blue. A glob of Bondo the size of a golf ball needs a bead of hardener about 1/2" long at room temperature. Heat accelerates the cure and cold retards it. If it has any hardener in it at all, properly mixed, it will eventually cure. The urge to over-use hardener is almost overpowering, get extra. Mix the hardener into the Bondo on a flat, throwaway surface until the color is uniform. You should have 10-15 minutes of working time, if not, adjust the quantity of hardener in the next batch. Mix only what you need, I always seem to end up with a polyester Popsicle of excess Bondo.

Fill dents until "proud" of the surface by 1/8" or so. A new, but rippled steel part must be "skim-coated" overall to a depth of about 1/16". Bondo is best applied with plastic "squeegees" available where you bought the Bondo. GO EASY with the stuff! It goes on deceptively easily, but you have to sand off any surplus! It is ALWAYS easier to put on another light coat than to sand down half an inch of pink granite. You have been warned!

When the stuff starts to cure (mixed that batch a little "Hot", didn't we?), stop applying immediately, don't try to stretch it.

A full cure takes about 24-48 hours, but Bondo is usually workable after 1-2. In fact, if left overnight, the stuff is too hard to work easily. You want it to just barely take an impression from a thumbnail. Work down the high spots with a Stanley "Sureform File." These look and work like a cheese grater. Regular files clog up, and wood rasps leave grooves. When you start getting close to the correct contour, switch to #80 sandpaper on a sanding block. Experiment with different blocks, from hard wood to softer rubber. A strip of 1/4" ply follows the contours of a tank well.

When the contour looks right, go to #180 and #220 to get rid of the sanding marks from the heavier papers. It may take several coats of Bondo to do this. Close your eyes and run your fingers over it. You can feel irregularities that you can't see. You can use an electric sander for some of this, if it has a fairly soft rubber pad. I've had good results with a Porter Cable model 333 Random Orbit.

OK, the contour looks great, but there will be small imperfections, tiny bubbles, and some scratches. Squeeze out a small amount of spot putty and put the cap back on the tube. Apply this to the imperfections with a flexible rubber squeegee, or whatever works for you. I use matchbook covers. Let it dry hard, maybe 2-3 hours, then work it down starting with #220, finishing with #400. Most of the putty applied should be sanded off, leaving only the little bit in the void or scratch.

The part is perfect! Couldn't be smoother or fairer, right? Well, as Senator Moynihan used to say, "If you don't believe the World exists to break your heart, what's the point in being Irish?" Spray the piece overall with primer, then wet-sand with #600. If you don't find more irregularities, you have a real talent for this and I hate you! You can use Bondo and spot putty in very light applications over the primer, then repeat.

Some people find this terribly frustrating; some find a sort of "Zen of Sanding." I'm about 50/50. You eventually find yourself with a part that shows no irregularities, even outside in sunlight, a uniform dull gray, smooth as a Vinyl Siding Salesman. It has taken some real effort to get here, but trust me, the BEST prep that you can do is just barely good enough. If you even think for a second that it needs one more pass, it does.

After this, painting is duck soup, money for jam, easy as cake, piece of pie. On to the painting article, knowing the hard part is behind you. Go forth and be joyful.

PS: The pic above ain't great art, but there's six inches of ice outside that door (winter 04/05).




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