While the frame is bare metal you need to closely inspect every inch of tubing and each weld to be sure there aren't any 'cracks' appearing anywhere. Sand or file away any weld splatter. Watch out for small 'spurs' created by bits of wire left over from wire-welded frames. You can safely grind or sand down any small spots in a weld run that are obviously much 'higher' than the adjoining sections of bead but do not grind or sand down any welds in an attempt to make them look smooth. There are often 'high spots' in a bead where the weld starts, stops or changes direction and these spots can safely be blended in with the main run of the weld.
Before you bother to do any frame prep it's a good idea to make sure you've mocked up the bike at least once, preferably several times and added any tabs, lugs, slugs or brackets that need to be welded in place. Working on the bike doing mock-ups while it's still grungy or rusty is perfectly normal. Just sand down any places where you'll be doing welding to bare metal. It's also a good idea to mockup both wiring and plumbing because you never know where you'll need a tab or bracket. Priming, filling and painting is the last thing you need to do on the bike.
Once you're completely ready, it's finally time to sandblast the frame, even if it doesn't show any signs of rust. If it looks relatively 'clean' it's still a good idea to swab the whole thing down with a chemical rust remover. Blasting or a chemical wash not only gets rid of rust but also provides a little 'tooth' on the metal for the primer to adhere to when you get to that stage. Keep in mind that even 'new' DOM tubing has a 'mill scale' on it but it looks nice and shiny. It still needs to come off for a good job to be done. Be sure to plug any threaded holes and the neck stem before you etch or blast.
While the frame is bare metal you need to closely inspect every inch of tubing and each weld to be sure there aren't any 'cracks' appearing anywhere. Sand or file away any weld splatter. Watch out for small 'spurs' created by bits of wire left over from wire-welded frames. You can safely grind or sand down any small spots in a weld run that are obviously much 'higher' than the adjoining sections of bead but do not grind or sand down any welds in an attempt to make them look smooth. There are often 'high spots' in a bead where the weld starts, stops or changes direction and these spots can safely be blended in with the main run of the weld.
On any parts of the frame made from 'flat' pieces of steel like axle blocks, motor and tranny mounts, wishbone reinforcement plate, neck gussets or control mounts look for 'sharp' edges and sand these to a very small radius, just barely knocking down the edges to prevent paint from cracking at the sharp transition point in the future.
Take a tap and run it down or through any and all tapped holes in the frame. You'll probably need both regular and 'bottoming' taps. Use a countersink to slightly bevel the edges of any holes in the frame tubing or various mount plates or tabs. This will help to prevent paint chipping or cracking at sharp transition points. Primers, paints and powder-coating do not like sharp edges or sudden transition points so even the very slightest bevel or radius on edges will improve surface finish adhesion significantly.
Use your bare hands to feel the metal. The touch of your fingers will often reveal imperfections your eyes will overlook.
Tools and Supplies
Tools of the trade for frame prep and metal work in general are pretty basic and most of the readers probably have most stuff they'll need already but for those just starting out this section of the article may be helpful.
For de-burring, blending and grinding nothing beats a small variable speed rotary grinder like a heavy duty Dremel or Foredoom tool and a supply of variously sized collets to hold mini-flapper discs.
Small pneumatic die grinders also work great if you can throttle down the air supply to keep the tool running at about half speed.
The key, whether electric or pneumatic, is to find tools that are as small as possible so it's it easy to get them into tight places.
Of course, you'll need sandpaper and those nice little flexible foam backed abrasive 'pads'. It should go without saying that you should use good quality wet-dry autobody sandpapers since the backing is flexible. Roll stock paper can come in handy as well.
You'll often need to plug holes to keep acids and/or finishes from getting inside and 'masking plugs' are readily available from a variety of sources. I buy from McMaster-Carr. Plugs made from EDPM are acid resistant but paint sticks to them, so you'll also need plugs made from silicone for use in painting as they're reusable.
Chemicals include Lacquer thinner, Metal-Prep and Acetone and of course filler putty and catalyst. A good rust remover is also handy to have around.
The reason I make a distinction between a rust remover and 'Metalprep' is because the two products are actually very different. 'Metalprep' is a brand name for a phosphoric acid formulated for cleaning and slightly etching metal prior to primer application. Most rust removers have a wide variety of other chemicals included in their composition, most of which you don't want on your frame or sheet metal prior to primer application. The rust removers have to be 'neutralized' and then thoroughly removed before anything else can be done. Metal prep on the other hand is applied and then washed off with water after about a two-minute etching time.
Finishing
Immediately prior to primer wash the entire frame with some type of 'degreasing and etching' solution. Follow the manufacturer's instructions to the letter. Contrary to popular belief etching acids like 'Metal-Prep' won't dissolve 'oils' like fingerprints and such. You'll see the acid turn 'whitish' where it's on clean metal, but it'll turn rusty colored where it tries to eat through grease or oils. For this reason, you need to always use a de-greaser before etching. A lot of painters I know use regular old 'Dawn' liquid detergent to wash down frames and sheet metal, but Dawn won't remove the residue of silicone-based products that might have been used on the frame in the past to prevent rust. For this reason, I suggest using any one of the commercial metal preparations washes specifically designed to remove both oils and silicones.
Do not use shop air to aid in drying the frame after washing as depending upon your filtration you may just be adding more oil back onto otherwise clean metal. I prefer to dry with an electric heat gun. Keep in mind that water can remain trapped in tiny crevices for hours so pay particular attention to drying around places where mounts, tabs and brackets have been installed. Also, inside threaded holes where some 'spray-can' computer cleaning 'air' can come in handy.
Once you're satisfied that the raw frame is as close to perfect as you can get it then go ahead and apply primer. Again, remember to plug any threaded holes and the steering neck 'bore'. From this point on never touch or handle the frame again unless you're wearing some of the disposable white cotton gloves sold at most paint or refinishing stores.
Rust will begin forming on bare metal in as little as 30-minutes even though it'll be invisible to the naked eye so don't wait very long before getting the primer down.
I would never powder-coat a frame and I suggest you don't either. Working on it later on down the road will be a nightmare and contrary to popular belief, rust does form under powder-coating; I've personally seen it on numerous frames and forks I've worked on in the past.
If you're not going to be painting the frame yourself then skip everything below this point as your painter will have his or her own methods to finish the job. On the other hand, you can give it a go yourself.
For the first coat of primer, I prefer to use is regular old rattle-can etching primer. I use either Sikkens or the products sold by Eastwoods. Two-part epoxy-based primers usually lead to problems if you ever need to weld-on a tab or something after you've premiered as successfully removing the product for spot welding and then blending in a new coat can be problematic.
Using regular etching primer allows you to 'spot sand' areas where you might need to do some additional welding.
At this point some folks will tell you that I've skipped a step. I haven't yet mentioned doing some frame 'molding' to smooth over welded places on the frame using body putty. (See the site section on frame molding for more information).
The reason for this is that materials and techniques have changed over time and builders are now tending to use etching primers first and then adding filler. It has been found that fillers, whether epoxy based or polyester based, can't make a 'chemical' bond to bare steel but they do chemically bond to primer very well. The trick is to sand the primer in the areas to be filled with 100 to 120-grit paper first then fill and mold as needed.
If you apply filler to bare steel, you need to get really aggressive in the immediate area to be filled and create a good 'tooth' for the filler to physically adhere to. Using 36 to 40- grit paper to really rough up the metal is standard practice.
To be honest I've done it both ways for decades and I'm starting to lean towards the primer-first school of thought nowadays.
There are also two schools of thought about etching primers. One school holds that you apply regular sanding primer directly over unsanded etching primer. The other holds that you finish sand the etching primer before overcoating with standard primer.
I personally finish sand the etching primer since in my experience the etching primer develops a thin surface finish after curing that is too 'slick' for regular sanding primer to bond with very well.
One thing is for sure and that is to use the same 'brand' of primers. Using Sikkens etching primer for instance and then overcoating with a different brand of sanding primer can sometimes lead to unfortunate surprises.
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