Stripping the Paint
From Bondline
Contents |
Introduction
Stripping the paint off of your plane can be a time consuming process. Why do you think it costs a minimum of $12,000 (2007 dollars) to have your plane painted? There are two basic methods for stripping and several variations for each. Some information is provided for all of these methods, but until someone actually goes down that path and shares their experience here, details will have to wait.
Chemical vs. Mechanical Stripping
The two basic stripping processes are chemical and mechanical. Chemical stripping is the application of a chemical to the surface of the paint. This chemical works it way into the paint and primer and causes swelling. This swelling usually causes the paint to detach from the underlying surface. Mechanical stripping is an abrasive process.
The basic chemical stripper for who knows how long is based on an industrial solvent called methylene chloride. Methylene chloride will burn your skin and is a known carcinogen. For that reason, safer but often less effective alternatives have been developed.
Chemical Strippers
Methylene chloride can be purchased at most stores that sell paint. Wal-Mart, Home Depot, Lowes, and Ace all sell the stuff. Methylene chloride-based strippers come in different grades. The more expensive the product, the higher the concentration (usually) of methylene chloride and the faster and better it works. The cost of the product can also be an indicator of the quality of the carrier (the gel). Since methylene chloride readily evaporates, the quality of the carrier is important.
Personal protection for methylene chloride requires chemically resistant gloves, other skin protection, chemical goggles, and a respirator. If you choose not to use a respirator, you will need extremely good ventilation. Goggles may not be necessary if you brush instead of spray, but there's always a chance of splashing or rubbing it into your eyes.
Safer alternatives to methylene chloride now exist. Some people swear by them and others think they are less effective. The most common one is the generic citrus strip, but Jasco makes a product that looks like green slime. These alternatives are usually more expensive, though. If using chemical strippers, you will have to balance safety with the size of your wallet.
Chemical strippers should not be used on fiberglass or other composite parts. The same action that causes the paint to swell will damage the matrix material in the composite either weakening the matrix and/or causing delamination.
Mechanical Stripping
Mechanical stripping can be used to strip the entire aircraft or as a finishing step. For most light general aviation aircraft, chemical stripping is used for the majority of the work. Mechanical stripping can be broken down into two basic processes, blasting and sanding.
First and foremost, blasting uses abrasive particles under pressure and can easily damage thin aluminum either through being overly abrasive, causing heat buildup (oil canning), or both. Second, you will probably not use a blasting process yourself. This usually requires industrial equipment. Some local car shops may have this equipment. If considering blasting, safer (not necessarily safe) methods include soda, walnut shell, and plastic media blasting. Plastic media blasting is now being used in military depot work and is supposed to be milspec'd down to 0.016 aluminum. However, some have indicated that plastic media should not be used on aluminum this thin. If considering this route, perhaps you should have the shop test their method on some scrap of the same gauge and alloy of the pieces you need stripped. See the maintenance manual for this information. Soda blasting can be messy and can get into every nook and cranny, but the equipment is typically portable. Like plastic media, some claim that it will damage thin aluminum.
The other basic form of mechanical stripping is sanding. This usually involves a dual-action or random orbit sander with different grits. However, another common method for detail work is to use Scotch Brite wheels on a die or angle grinder.
Mechanical stripping should be the primary method for composite parts.
Chemically Stripping Your Grumman
Before starting anything, you have to decide whether you want to paint your airplane in pieces or as a whole. If you disassemble you plane, be forewarned. You and your Inspection Authority (IA) may find costly problems that have been hidden for years. Disassembling your plane may also require you to manufacture things to hold those pieces such as sawhorses for the wings. However, disassembling your plane is usually the easiest way to paint your plane and end up with the best job possible.
Before you start anything, try to wash as much dirt and grease off of your airplane as possible. Not only will this allow the chemical stripper to do its job better, but you'll need to do it for this next Grumman-specific step ...
Because chemical strippers will attack just about any kind of adhesive, the bondlines need to be taped off with aluminum/foil tape. Make sure the surfaces are clean or the tape may not stick properly possibly allowing stripper to get to the bondlines. The manual does not say so, but you'll need to tape/mask off anything else that you don't want the stripper to get into.
Before applying the stripper, you will want to have several items ready. These include, but are not limited to, the following:
- Plastic drop cloths of at least 1 mil maybe 2 mil or higher for laying over the wet surfaces. This keeps the stripper from evaporating.
- Drop cloth made with Tyvek. This is for the floor. It will keep stripper and paint off the floor, and the Tyvek cloths are a lot less slippery than plastic. One Tyvek drop cloth will last the entire job it's that tough.
- Plastic strip tools. Not all of the paint and primer will lift. Metal strip tools will damage the aluminum.
- A good supply of green (fine) and grey (very fine) Scotch Brite pads. When soaked in stripper, these will be used to remove any remaining paint or primer "haze". Scotch Brite pads are available at Lowes or through Aircraft Spruce. The red (medium) work as well, but some local hardware stores don't carry the red. Red and green are good for paint and primer, grey is good for that primer "haze". Use the finest grit possible to remove as little aluminum as possible.
- One gallon milk jugs with the tops cut off or other containers you don't mind throwing away. These will be used to hold and dry out the used stripper and paint to make disposal easier.
- A 5 gallon bucket with soapy water and a sponge with Scotch Brite on one side, maybe a good supply of paper towels. These will be used to wash any remaining stripper and paint off of the surfaces as the last step.
When applying the stripper, you need to brush in one direction. Read the instructions. It has to do with the carrier/binder developing a film to hold in the methylene chloride. You want to be generous with the stripper, too. Most require a thickness at least equal to the underlying paint and primer. You may want to purchase some plastic drop cloths to lay over the stripper to help keep it from evaporating. Imron and the red epoxy primer that is typically underneath are some tough stuff and usually require a longer "soak" time.
If the stripper doesn't lift the paint and primer, then some mechanical action will be required. Do not use a metal strip tool. It will damage the aluminum. Buy the plastic ones at Wal-Mart, Home Depot, or Lowes. If the factory did good prep work, then usually some scraping is required. If the primer was thin or non-existent, then the stripper will probably lift the paint right away, and it will slough right off. Scrape the stripper and paint into a plastic container to let it dry out. Once dry, it will come out as one piece that can be thrown away, and you can reuse the container.
Usually some final finishing steps are required. This is where the Scotch Brite comes in. You will need to dip the pads into stripper and then scrub any remaining areas with paint or primer "haze". Scrape the slurry off or use paper towels. Follow up with the soapy water to remove all stripper residue. Use the Scotch Brite side first to break up the gunk on the surface, then use the other side to remove the water. Follow up with paper towels if desired.
Bondlines and Finishing Work
Once you have determined that you are done with the chemical stripper for a given component, pull the tape off of the bondlines. If you are lucky, a lot of the paint will come off with the tape.
You will now need to sand the paint off that was under the tape. One method that works well and quickly, but requires some practice first, is a Scotch Brite wheel on an angle grinder. Because angle grinders develop very high RPMs, even a fine grit (green) can easily damage the aluminum if too much pressure is used or if left in one spot too long. Working one area too much may also cause heat buildup and may damage the bondline. Start with a coarse (brown) or medium (red) wheel to get through the Imron, then switch to the fine wheel (green). If you start with the green wheel, it will load up quickly. It can not be stressed enough to practice, especially occasionally feeling the surface to gauge how much you are heating it up. If it burns (on your practice piece), move the tool more quickly. Use a forward and back motion as you move the tool left to right. The Scotch Brite pads don't last forever, and once they begin to wear it's better to get a new one. One they begin to wear, it takes longer and more heat is produced.
To get paint out of or near lap joints, you can use the edge of a fresh Scotch Brite wheel with light pressure and, for even harder to reach spots, a stainless brush attachment for a Dremel tool.
Once done with the detailing for that component, you may want to wash the surface again. If doing this work around other components, wait until they are all done.
Supporting the Wing and Fuselage
The maintenance manual illustrates the lifting and shoring points, which are at the firewall and near the back edge of the honeycomb box portion of the fuselage. However, the manual does not show how to support the wings when removed or how to strip the bottom of the fuselage.
In the pictures above, the wings are supported with padded sawhorses. When the wing is right-side up, the relatively flat bottom makes it easy to support. Even so, you want to position the sawhorses so that they are supporting the wing ribs. This is more critical if you have to flip the wing over since a relatively small portion of the rib is supporting the wing weight. Here's where good padding makes a lot of sense.
To get to the belly of the fuselage, it is better if the fuselage is flipped onto its side. It is recommended to do this only if everything has been removed from the fuselage so that it is just a shell. It is fairly light and handle be lifted by two people, but you'll need a third person up front to help rotate it because the fuselage is wide at the firewall. The fuselage can safely be supported by its two ends, but you can also add a sawhorse in the middle. If you do this, brace the fuselage internally at this location with a 2x4 with plywood on either end to help distribute the load. You will also want to have some way of adjusting how much weight is on each support. With just a shell, the fuselage CG is near the middle sawhorse, and that's where most of the weight is. By putting some addition padding under the fuselage aft bulkhead, the weight will be more evenly distributed.
