When are you Finished Sanding and Priming Bike Parts?

[SteveHGraham]

Since I got good input on the HVLP question, I'm going to see what else you people know about painting.

My two side covers have been primed repeatedly. I did the crude sanding with 180 grit, and then I primed them again and went over them with 600 grit. Now I am having a problem deciding when I'm finished.

The parts are not slick factory side covers. Some guy who sells aftermarket parts makes them from fiberglass. The edges are a little rough.

The covers have tabs that project upward, and these fit under the bike's seat. They also have three flat sides which are not too easy to see when they're installed.

The tabs and the flat sides are not as nice as the outside surfaces. I used a guide coat, and there is still a fair amount of guide paint on the sides and tabs.

Should I quit worrying about areas that aren't easy to see?

I'm also wondering what to do with the edges. When I sand the flat sides up to the edge, the primer comes off the edges quickly. Obviously, if I re-prime the sides and sand them again, in order to get primer on the edges, I will have to sand the sides again. It seems like this is likely to end in an endless cycle of priming and sanding, because every time I sand, I will remove some primer from the edges.

What's the best way to deal with this? I'm thinking maybe I should mask off everything except a little strip of the edge, apply primer to that little strip, and then mask it while sanding the sides. I would end up with 1/4" of rough paint all the way around each cover, but at least I would be finished, and it would be uniform.

I am attaching the only photo I could get my phone to take. The view is from the front of the cover. The tab is the mess to the left.

[Hopefuldave]

HI Steve,
I'm not really the guy to answer this, as I'm a bit fussy...

I'd use "high build" primer, also known as "spray putty", it gives a way thicker coat and dries quite fast - the coat's thick enough to fill most of the pits I can see, although you may have a lot of rubbing down to do! It's a lot softer to sand than regular primer (not quite as soft as bondo/filler, but close). It's usually coloured "a mixture of puke green and yellow", to quote one of my fave fillums

When you sand, are you using a backing pad? Hand-held wet and dry paper will follow concave curves well, but will cut more on edges, like you're seeing... A very stiff foam sanding pad will make a big difference, for serious flatting on *flat* panels I've used 1/4" steel or aluminium plates, particularly on thin plastic/fibreglass panels that can flex under pressure, followed by the backing pad going curved...

For the join between panel and tab, if you're picky like me, cut a suitable curve/edge in foam and wrap the abrasive around it, again i've sometimes used a suitable steel rod as though it was a deader than dead smooth file for work like that.

[BadDog]

For me, the primer doesn't start until the part is already smooth and "right". For fiberglass parts that are pretty close but with uneven surface and pits/fibers to deal with, I would probably start with one, maybe two applications of polyester filler. The first pretty much sanded off to provide a level even surface, the second very light and sanded smooth to prevent edges. That second coat could easily be substituted with a high build catalyzed primer, but the main point is no sand through if possible so you don't have edges. Then the primer starts for filling scratches and near invisible imperfections, potentially even air bubbles or sand-throughs that may have gotten in the final polyester prep coat. I can't imagine needing more than one coat of the cartelized primer, two if I found I had done a less than ideal job on prep. So in general, my response would be that you likely went to primer too soon and didn't sufficiently prep for primer.

[steamin10]

(bad dog got me!)

IMHO, when I have rough parts, I violate the edges until I am satisfied that they are the shape and smoothness I want, right off. then a dark or light gray, whichever is handy, and lightly sand it, to find the imperfections and pinholes in the surface. These are filled with a bit of spot putty, red or blue, and left to dry. Then the wet sanding begins with flex foam block and 220 and finer grits until you are surface smooth. The second guide coat will show you what is happening with your paper. It is about seeing what you have left, as the colors pop out of the surface. A final thin blast of primer to even the top out to the eye, and final light scuffing for paint adhesion, and then hang and shoot your color.

You are the master here, and you will control all that it becomes. I have done pieces several times to correct flaws at times, bu t it is the process of learning that is the goal. this is just one project. Dont let your hair fall out over it, do it and take the next step. You can be a fair judge of the results, and reject it if need be. So satisfy yourself, and dont create failure from indecision. Carry on.

[SteveHGraham]

Thanks for the help.

Some of the pitting is around the bolt holes, and it's in the gelcoat. I am not going to bother fixing that, because the screws will chew that area up anyway. The other pits are in primer, not gelcoat. The primer goes on pretty thick. I used spot putty on the gelcoat imperfections, apart from the ones around the bolt holes.

The primer I got is supposed to build well, but the Sherwin-Williams dude did not actually refer to it as spray putty.

I am using a foam pad under the paper. I didn't think of rolling it around a different shape. I appreciate that tip.

The solution I went with was to leave about 3/16" of the the edge rough. I've seen parts that have rough, masked-off edges from the factory, so I think this will be acceptable. I masked it off and applied primer with a brush. When I do the remaining sanding on the parts, I'll cover the rough area and sand the rest.

One of the covers is good enough to paint right now, as far as the shiny part goes. I did some more sanding after posting the photo, and I exposed some of the gelcoat on the other cover. It needs one more blast of primer.

I have a knack for picking deceptively hard jobs, and my teaching materials didn't really prepare me for this. The DVD I learned from shows a guy doing tanks and fenders. They're big, so they can be mounted on stands, which makes sanding and painting easier. They also had no tricky parts, so he didn't say anything about handling difficult shapes. You would think painting two side covers would be easier than a big fender, but I am having problems you don't see with fenders.

[BadDog]

I've used wood blocks, sections of rubber hose, whatever provided the contour needed.

Also, be careful of classic lacquer "spot putty". It can be glassy smooth and perfect, two weeks later you notice it shrunk in and scratches are visible. It MUST be put on thin, and given adequate time to dry, the thicker (or larger pit) the longer you must wait (or use lights). The 2 part polyester putty works almost exactly the same way, the same general smooth consistency, but goes off in under an hour (depending on hardener, temp, humidity) and it never shrinks. But contrary to what some folks I've seen do, it is NOT a true filler. You use it for about the same category of stuff as classic spot putty, maybe a bit more. But when built up thick, it tends to crack. For that, a variety of actual fillers (aka "Bondo" re "Klenex") are available, including some with structural stranded components (fibers) for more strength. Horses for courses...

[SteveHGraham]

I am not too thrilled with the Bondo-brand putty I got. I had read about shrinking problems. The pits in the gelcoat were microscopic, except for one crater, so I figured it would work. It seems to be okay in the really tiny holes, but two applications did not really fix the big one. Thankfully it's in a place where it won't be visible.

These parts are not the greatest substrate, either. The guy who makes them obviously made a mold from some used Moto Guzzi side covers, and the molds were not perfect, so I started out with some issues a factory part would not have. They combine the gouges in the original parts with the errors in the molds.

The factory covers cost $115 each, and they are not nearly as sturdy as a Frisbee. Pure plastic, with no glass, maybe 3/32" thick.

A real man would get an English wheel and some steel sheet and make real covers. If only I knew a real man or a place where I could hire one.

[BadDog]

To recap, If I were doing parts as I interpret your description, basically already to proper shape but a bit rough with pits and minor imperfections here and there. I would have started with the polyester putty and covered the whole thing, then with relatively coarse paper (maybe 180) sanded the whole thing till the gel coat started peaking through all over, sort of like spotting for scraping. At that point you should have a basically uniform appropriate surface, and you from that you could begin what you've done. The catalyzed primer will easily handle 180 grit scratches without shrinkage issues. If using lacquer primer, I would have finished with 270 or 360 or so (not even sure I remember the grits right). Or, if you detect flat spots and imperfections (you learn to "see" with your hands because the eye is easily fooled by blotchy surfaces) you may need another coat of polyester putty before moving to primer. But you never move to primer until the part is nearly perfect and "done" without it. Polyester putty is not expensive in modest amounts, and is easy to use. I believe that missing that initial prep stage cost you a lot of trouble, and may cost you in final satisfaction. If you try that approach on your next painted part, I think you'll find good results far faster and easier.

An English wheel is not the answer either. It's another tool to make things easier, but certainly not required, somewhat like a power scraper to hand scraping in that application without basic supporting skill/knowledge is looking for disaster. And particularly with deep crowns like your parts, you're looking more for a planishing hammer/dolly. And it could all be done by hand with a shot bag, proper sheet metal hammers, and a selection of high/low crown dollies.

[SteveHGraham]

These parts are pretty sturdy, so if they look crappy, there is enough meat to redo the job. I can always start over and hit them with Bondo from the get-go. It's too bad I'm on the Internet where I can't show things to people up close. I think they're okay now, but I won't know for sure until the paint hits them.

Anyway, live and learn. Eventually the bike will have two side covers instead of the one it currently sports.

One lesson I'm taking away from this is that you should never buy an Italian motorcycle unless you're willing to suffer for it. If this thing was Japanese, I could fix this issue on Ebay for twenty bucks.

[BadDog]

They may be fine, and that definition is totally up to you. I would say just get yourself satisfied, spray the final, and find out. You may mess up the final, and that may prompt you to start over, or just sand it and do it again. Either way you are learning and next time will be better.

I started body work with NO help and no training (much like machining) when I built my first car. No internet forums then. Dirt poor and living in N Alabama, and no way I was getting a car bought for me when I turned 16. Much like I bought my first "real" motorcycle (a Honda 125) at 12 years old in a box at a yard sale for $20 (riding it a few weeks later), I bought a '70 Cuda with no engine or trans with a crumpled front right fender for $70 delivered from a local junk yard. I couldn't find/afford a fender, so I beat out the one I had and eventually borrowed a gun to spray the whole thing "candy red" (not really, just bright red enamel single stage of course... <sigh>). Got a 340 magnum from a balled up Duster, a 727A from a Polara(sp), and a "bump stick" complete with tunnel from a Challenger to replace the cut up trans tunnel where someone put in a 4-speed using a hatchet (or so it looked), My 16 year old self was very proud of that car, but looking at old pictures years later, that fender was EMBARASINGLY bad. Roughly 10 years later I was repairing high end European sports cars and charging $3k and (way) up to spray paint. I never went to body work school of any kind (though I did get invited/comp'd to go to Sherwin Williams "Advanced Painting Technique" school in Atlanta for a week, but that was more to get me to buy their paint system for the shop). And I had trade school graduates that I wouldn't let in my shop. But you gotta start somewhere...

[steamin10]

BadDog: Your learning curve parallels mine in much the same way. Bikes started later when I was out of Mom's house, as she had low opinions of motorcycles. I fooled with fender changes and spotting in various damage on friends cars. Then my dad repaired his favorite Gallaxy 500, that now had every color door and fender panels. He bought a small air compressor for me, and I had the gun, and away I went with fire engine red, on the former Navy Blue car. It was a trial, as all the trim came off, the jams spotted, and hood and trunk sprayed for continuity inside. It came out with a bit of peel, and I was aware of some things that could have been better, but Dad could not keep quiet about how good I had done. (It was apinted on a calm 75 degree day, under the trees) It was good, but I did it better as time went on. I worked in a commercial shop for 3 years, and then life took another turn, and I did work at home, in my nonexistant spare time.

Good advice from you, I concur with your vision. I bid you peace.