78 GS 750 Rebuild

[nvr2old]

Glad to see you're jumping in with both feet, Sam. You created a lot of work putting that much filler on, but hey, ya gotta learn somehow. Seems your first problem may have been the amount of cream hardener. Temperature rules the amount needed. Must have been pretty warm for it to have kicked off before you even had it all applied. "Sculpting" really works if you can start cutting the filler back before it totally kicks off, when it's still rather soft. It's easy that way with long gentle strokes. Are you using 36 grit on a block to cut it back with..? If not, that's the ticket. All techniques that come with experience. I commend you for your effort. You'll be glad you did it when it's all said and done.

[Sam 78 GS750]

I did cut back on the hardener a bit, but it seems not enough. I read in one of your old paint threads that you would start sanding while it was still soft, but by the time I did the last section/batch the first section was already rock solid. It has been pretty warm, but I have an AC unit in the garage and I keep it set to 76. Not enough to stay cool, but enough so that it's bearable to work without killing the electric bill. All part of the learning curve I guess, but now I know to use a lot less hardener than I have been, especially when it's hot out.

As far as the sanding, I've been using 80 grit. I tried 36, but it was a bit difficult to work with. I was probably using too much pressure though. As far as the block itself, I have one of the 3M hard rubber ones, but I feel like it's not flexible enough to hug the contours of the tank. Am I right in thinking that? I've been using stiff sponges instead, and when they've lost their grit just wrapping paper around them... Not sure if that's going to effect the results.

Also, I'm not sure how to know when I'm done sanding. Sounds like a weird statement to make, but am I going for mostly metal with spots of filler or the other way around?

[nvr2old]

Convincing people to begin with coarse sandpaper is always hard. Starting with 36 grit is truly the way to go in the beginning. 80 just won't cut the filler quickly. I'll bet you found yourself sawing away at it for what felt like a lifetime. 80 grit is basically just for smoothing out the 36 grit scratches a bit. The 36 is what shapes it, and shapes it quickly and cleanly. Your hard rubber block is the correct tool, also. The sides of you tank are pretty flat as well as the top, so a hard block works nicely. The more flexible ones will work for block sanding the primer, but to shape the filler, you were using the right one. Overall your tank looks good for what I can tell in your pics.

The only area I can see that might need a bit more filler are the 2 small spots on the left side in front of the emblem mounts. W/o being able to run my hands over them, they appear to still bit a tiny bit low. If they look like a solid circle instead of being more feathered out, chances are you might have taken a little bit too much off, which is easy to do, but will show up as a ring when it's primed. Otherwise, carry on..it's looking good.

[chuck hahn]

And an RO electric sander does wonders till it hand sanding time

[Sam 78 GS750]

I was definitely sawing away for what seemed like a lifetime. I did hit it initially with the RO sander to knock down the big lumps and lines, but then I started to hit metal in some spots while spots next to it were still super thick so I stopped. The spots on the left were definitely solid circles and not feathered, so I did take too much off I guess. The hardest part to sand/shape seems to be the rounded areas at the front of the tank. Again, I'm hitting metal in the middle of the tank, but the edges are still super thick. I'm trying to smooth out the surface of the metal, not reshape it completely, but it seems like that's what I'm unintentionally doing.

I realized one of my big mistakes. I watched a bunch of videos on youtube before starting. In one video, the guy just dumped the entire pool of filler in the middle of a door panel, and spread it all around the door from there. While that might work for a large relatively flat surface like a door, I think it was the wrong approach for a bike tank. Even though I worked in smaller sections this time around, I still dumped each whole batch of filler on and tried to spread it around. Since I also added too much hardener, it was setting up before I could really work it how I wanted to. That's how I ended up with it on there so thick.

I think I also finally got the "long broad strokes with a light touch" through my head. Probably why it's still super thick in some spots and barely there in others.

[nvr2old]

Your tank was in pretty good shape before you started on it. A skim coat after addressing the small dents is to fill the shallow areas like on top and the rear mount area. If it all sands down to bare metal except for those low spots, that's a good thing. It shows you it doesn't need any filler. The less filler the better. The tank "tells you " what it needs and when it's done just by how the filler comes off. It takes awhile to learn the language, so to speak, and you're doing fine.

[Sam 78 GS750]

I'm starting to learn the language a bit. Not well enough to carry on a conversation with some locals about the dangers of inflation and the looming possibility of having to devalue the currency...but enough to order a beer, ask where the bathroom is, or get directions to the museum.

I may do one more skim coat with a lot less hardener, or at least hit the spots where there's filler left after sanding (and go a little outside them to be able to feather the edges). Since I put it on so thick the last time, there are air holes everywhere. I also took too much off in spots where filler was needed trying to sand down the areas where it wasn't needed, but still on super thick. Since it was on so thick and uneven, I couldn't really do long broad strokes with the block. I had to focus on small areas.

How do you handle the seams and the sharp curve leading into them all around the edge of the tank? I tried to avoid it but I got filler in there. Sanding those areas with the block doesn't really work. I just took it down to metal there with bits of paper folded up. Is that what you do, or do you fill them and then just follow the contour of the tank down to the edge?

[nvr2old]

There's a fine line between the right amount--not enough--too much hardener. Too much as you found, hardens way too fast, not enough, and it will never harden (which is way worse), and you'll have to literally scrape it off and start again. You can hand sand around the seams. Some areas just require that. In an earlier post you described a YouTube video where the person poured the filler into the middle of a door panel and then spread it out. Not really a good technique on a bike part. Just scoop what you need onto your spreader and squeegee it out over the area as tight as you can but with as much as necessary. That will help distribute the filler more evenly, and hopefully prevent the amount of buildup you experienced. Better to go with several thin coats rather then glomming a bunch on at one time. If you keep spreading it over areas that don't need it, you're just creating extra work, using up expensive materials unnecessarily, and with all the extra sanding, sanding away areas that are already filled correctly. From what I can see, you're 95% there, it doesn't require a "hard-shell coating" over the entire tank. High-build primer will also fill a LOT of shallow areas with a couple of applications and block sanding sessions.

[Sam 78 GS750]

After that last bit of advice I decided against trying to skim coat the whole tank again to keep trying to "get it right". I just filled in the air holes and touched up some problem areas. I didn't get it perfect, but I got it the best I could with my current level of ability and experience.

To be honest, I probably could have done it five more times and I still would have made some of the same mistakes. I had to see it with primer over the filler to realize a couple of things I should have done differently when sanding the filler. When I was block sanding the primer, it was a bit of an "ah-hah" moment. The primer showed me what the filler should have looked like when I was done.

The filler "done". Before priming I didn't see some mistakes. Now after blocking the primer, I look at these pics and a couple of spots jump right out at me. Live and learn...
20180825_171306 by samL9, on Flickr

20180825_171330 by samL9, on Flickr


Etch primer on there.
20180826_023232 by samL9, on Flickr

20180826_023240 by samL9, on Flickr

20180826_023249 by samL9, on Flickr


If you look at the line/contour where the side meets the top of the tank on the left side, near the bottom/front of the tank, you can see one of my bigger mistakes. Instead of one crisp line, there's a bit of a flat spot and two lines... Not sure how to fix that at this stage, or if I even can fix it.
20180826_023258 by samL9, on Flickr

and the primer surfacer
20180826_032129 by samL9, on Flickr

20180826_032137 by samL9, on Flickr

[Sam 78 GS750]

This is after block sanding the primer. The guy at the paint store put a blue tint in the etch primer. I wouldn't have thought to ask him to, but it was super helpful to see when I needed to stop sanding. When gray turned to bluish gray, I knew I was about to go too far. Larry, what you told Pete in his Kat thread about shoving primer in pinholes with your finger saved me at least one round of sanding.

20180826_165314 by samL9, on Flickr

20180826_165322 by samL9, on Flickr

Some of those "rings" Larry was talking about where I sanded a bit too much filler off. Even after going back over those spots, I guess I still sanded back too far.
20180826_165247 by samL9, on Flickr

20180826_165307 by samL9, on Flickr

I'm thinking 2 more rounds of block sanding should do it. I re-primed already. Not going to post pics, because they look the same as the ones I already posted. It's still a bit rough, so I'm thinking 180 grit again. After that another round of primer and then 400-600.

We'll see what it looks like after this round of block sanding with 180 and I'll take it from there...

[nvr2old]

Your parts are looking better all the time. Don't be afraid to continue block sanding until you break through to bare metal. That will give you the optimum effect of what block sanding does. All the guide coat needs to go away. It tells you when you're done, basically. Those "rings" will start to settle out with more sanding. Hitting bare metal around them is when you've gone as far as you can, then stop. Same with your upper body lines, which look a bit sharp to me. I would try to soften them a bit with your next round of primer blocking which should then more match the area you said didn't look correct. 180 again is a good choice. It will cut clean, and quickly. Then go one more round with 400 and then just a light 600 to ready it for base coat. It's looking better then you're giving yourself credit for. Wait till the next block. The difference will be very noticeable.

[Sam 78 GS750]

OK, I guess I could have gone a lot farther on the last round of sanding then. As soon as I hit bare metal in a tiny spot on the sides, I stopped. As soon as I saw the etch primer peaking through on the top I stopped. Whatever guide coat was left got primed over again...

I re-shot etch primer on the bare metal spots because that's what the data sheet on the high build said to do. I was trying to avoid having to do that, but now that I think about it shooting etch primer really quick in spots before shooting the high build is a lot easier than extra rounds of priming and sanding and will save a lot of material in the long run.

The body lines look a lot sharper in the photos than they actually are because of the guide coat, but they are still pretty sharp. I've been worried about sanding them directly and messing them up... I've kind of been sanding towards them from the top and the sides, but never quite hitting them. They feel pretty rough doing that though, so this time around I've been going over them a bit more directly to smooth them out.

I'm also still figuring out the best technique for blocking the front. The top and sides are flat, so a lot easier. The front is a lot rounder, so I'm not quite sure what to do with the block if that makes sense...

As always, thank you for the help and guidance.

[nvr2old]

A small soft rubber block would work for the front section. They're cheap to buy and very efficient. You could also cut down a wooden paint stick and wrap it a few times with whatever grit you're using. Just criss-cross your sanding motion and you should be fine.

If ever I start to sound invasive in your process, please tell me.

[Sam 78 GS750]

Not invasive at all. You're advice and suggestions have been invaluable!

Especially this bit right here...

Don't be afraid to continue block sanding until you break through to bare metal.

How did you know that's what I was worrying about?
As soon as I stopped being concerned with hitting bare metal and went to town, things started looking really nice, really quickly. You typing that one sentence probably saved me hours of work and a ton of primer (and probably a trip to the store to buy more when I ran out after wasting it lol).

Oh, and a smaller block was definitely what was missing for up front...

I think I'm done sanding, but that's what I thought last time around. Maybe you can take a look at the pics and let me know if you think I should take it down a bit more? Seems ready to prime again and then block with 400 though.

Here's how it looks now. What looks like guide coat lingering in spots still is actually etch primer and bare metal/filler peaking through. Except for the tank badge areas where I masked for the screw holes with the filler. Not really worried about those low spots though.
20180828_181216 by samL9, on Flickr

20180828_181227 by samL9, on Flickr

20180828_181748 by samL9, on Flickr


I do have a couple of pinholes I need to take care of still.
20180828_181653 by samL9, on Flickr

Up close and magnified
20180828_181619 by samL9, on Flickr

number 2
20180828_181707 by samL9, on Flickr

20180828_181729 by samL9, on Flickr

Not quite sure what to do with those... Glazing Putty?

[nvr2old]

I think I would fill those pinholes with a bit of plastic filler. They're more then what primer will fill. You could also take your block and criss-cross back and forth across (not with) your top body lines, too. It seems to me like they could still use a bit of softening. Other then that your tank is looking very nice. Feels good doesn't it?