Bodywork 101 question

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I restore old woodworking machines in my spare time and am looking for some pointers for a smooth metal finish over a weld.  Usually these 50 year old plus machines have tons of errant holes from electricians and non-factory attachments.  What I am doing is TIG welding the 12 gauge sheet metal holes flush, grinding down the welds with an angle grinder, than smoothing them with a 80 grit disk.  From there I fill with bondo creme filler and sand with 220 grit in a 5" ROS sander.  I can see that the repairs are not totally flat and sunken in a little.  I applied 3 coats of filler in this latest case.  What am I doing wrong?Should I be sanding the area with a large flat sanding block to prevent the divots from forming?Chris
Reply:Yes, a sanding block will reduce the pocket. Also, on bigger holes, or large patches, you can shape or rasp bondo quickly. Regardless of how much hardener I use, when it comes time for a finish pass, I let it set up for a few more minutes, then it is much more accurate to sand out. That should take care of the problem. Good luck, Paul.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:12g ?Should be able to fill the hole with Filler rod and grind flush w/o any Bondo.Ed Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:Pics of the repairs (pre grinding) would help see what's going on, but it sounds like you're getting the area too hot, allowing it to sink lots. Should need very little, if any bondo.
Reply:I will post pictures tonight.  My last repair was a 3/4" hole in the corner of a cabinet.  The corner has about a 3" radius.I am able to build up filler proud of the hole, but when I smooth it with the angle grinder I end up creating a slight divot.  I am not putting more than 1/16th inch of bondo over it, but with the RAS I end up sanding a divot.  I guess what I am looking for is two bits of advice1 - how do I grind down the weld without creating a divot with the angle grinder, should I use a belt sander?2 - how do I smooth the bondo without creating a divot again?  Should I use a really large flat hand sanding block?Do I just need more practice?I will post some pictures of the finished work tonight.Chris
Reply:Originally Posted by hotrodderPics of the repairs (pre grinding) would help see what's going on, but it sounds like you're getting the area too hot, allowing it to sink lots. Should need very little, if any bondo.
Reply:Skimcoat the entire piece and block it out with the longest block you can fit!
Reply:The angle grinder with a grinding wheel is far too aggressive. Try a flat sanding disc, some like a flap disc, I like the flat ones, they're cheaper. An 80 grit flat disc will remove metal and leave a better finish.One other point, what is a divot? I don't golf, is that a hole from swinging at a golf ball?City of L.A. Structural; Manual & Semi-Automatic;"Surely there is a mine for silver, and a place where gold is refined. Iron is taken from the earth, and copper is smelted from ore."Job 28:1,2Lincoln, Miller, Victor & ISV BibleDanny
Reply:re: what is a divot.  In golf, it is the lump of turf dislodged by the club when trying to whack the ball.  In general usage, a sunken or depressed 'dimple' in a surface.   But usually meaning more than a small smooth dimple, and more of a hacked out area, roughly circular in shape.  If it is big and irregular, I'd usually call it a gouge.  If it goes all the way through the material, it is a hole.  If it is a machined hole that only goes partway through the material, it is a blind hole.  Etc, etc, etc.re: the divots or depressed areas.  If you are going for an automotive-level paint finish, then Bondo after the metal repair and use sanding blocks to sand smooth and level.  But on 12 gauge steel, you should be able to get a 'machinery'-level finish by filling the holes with weld, and then sanding with flap discs of various grits.  A few coats of sandable primer, feather sand the edges into the old paint using sanding blocks again, spray topcoat(s).Don't use the grinding wheel for more than hitting the very tops of the weld bead, if that.  Use a flap disc instead.With a little practice and skill, you can sand/grind a weld bead so that you can barely tell it was there.  YMMV.And yes, practice.  Take a piece of scrap 12 gauge, make a small weld on it, then sand/grind it flush with some flap discs.  Repeat.Or for flat areas, you can use a belt sander.  With a zircon belt to sand steel.  And make sure there is no saw dust left in the machine, and no cloth sawdust bag.  Pretty much if you use the machine for sanding metal, it is really no longer for sanding wood or other 'soft' things.  The best laid schemes ... Gang oft agley ...
Reply:Originally Posted by tanglediverThe angle grinder with a grinding wheel is far too aggressive. Try a flat sanding disc, some like a flap disc, I like the flat ones, they're cheaper. An 80 grit flat disc will remove metal and leave a better finish.One other point, what is a divot? I don't golf, is that a hole from swinging at a golf ball?
Reply:Tilt the grinder up steeper. Only bump the weld. Use a straight edge to ID the high spots. Mark them, I use a yellow carpenters crayon. Bump off the color. Straight edge, yellow, bump, straight edge, yellow, bump, etc. Slow down, you're in too much of a hurry. Buy the finest grind wheel you can find; I think mine is a 36 grit. Then use a fine flap disk; WITH PATIENCE.9-11-2001......We Will Never ForgetRetired desk jockey. Hobby weldor with a little training. Craftsman O/A---Flat, Vert, Ovhd, Horz. Miller Syncrowave 250
Reply:Originally Posted by Craig in DenverSlow down, you're in too much of a hurry.
Reply:I don't use bondo. Just wire and flat grind. Course Cat yellow paint is very good.
Reply:Originally Posted by *chris*Wow, in only 50 posts you figured out I am inpatient?Chris
Reply:Ok, here is my painted repair, I went crazy with the contrast to really show the bumpiness.  This was a 1" diameter hole that I tig welded shut.  I think I need to sneak up on flat with something less aggressive than a 24 grit grinding disc, it cuts sooo fast thoughThis is what the whole machine looks like.  The image above is a blow up of the area to the left of where the wire currently enters the cabinet.
Reply:24 grit!?!?! I cry foul!! SLOW down!! 9-11-2001......We Will Never ForgetRetired desk jockey. Hobby weldor with a little training. Craftsman O/A---Flat, Vert, Ovhd, Horz. Miller Syncrowave 250
Reply:Chris;  Go to where you buy the bondo/body filler, and ask for spot putty and spot putty applicator. The red oxide spot putty is the best. And use a paint stick to wrap the sand paper 500 grit 220 at the very least. This lets you get a wide area smooth [no divots].Be sure to let the bondo cure completely before applying the spot putty, primer the bondo [for best results] and the spot putty must cure completely as well before primering and painting. Also if you wet sand the primer after spot putty [if you use any] or just the spot putty you wont be able to see any blemishes.    Auto body work for past 30 sum years talking here.Miller Thunderbolt 225Millermatic 130 XPLincoln HD 100 Forney C-5bt Arc welderPlasma Cutter Gianteach Cut40ACent Machinery Bandsaw Cent Machinery 16Speed Drill PressChicago Electric 130amp tig/90 ArcHobart 190 Mig spoolgun ready
Reply:Slow down with the grinder and sneak up on the desired fit/finish/level.  You're being to agressive and fast with the grinder.And/or, for flat surfaces, use a belt sander with a good zircon metal-work belt.  Just realize that sanding metal will be tougher than sanding wood or Bondo, both on you and the sander.Or if you want to use the Bondo, use a bigger flat block to block-sand the filler.  With Bondo you should be able to make the repaired area pretty much undetectable to the eye.  The best laid schemes ... Gang oft agley ...
Reply:I would not recommend Bondo. Evercoat has a better line of fillers. They go on a lot smoother and are a lot more durable than Bondo when cured.-Matt------------------------------------------
Reply:Originally Posted by STwelderThe red oxide spot putty is the best. And use a paint stick to wrap the sand paper 500 grit 220 at the very least.
Reply:Originally Posted by *chris* ...I am only using the premixed red spot putty in a tube.