Before we begin, it’s important to understand how the prototype will be used and determine the level of finish accordingly. From there, we have many different techniques to finish, prime, and paint the prototype to the desired properties needed. We’ll go through some of the different steps in this process to go from parts off the machine to a smooth, primed part ready for paint.
It’s not always how you start that’s important, but how you finish is everything!
What do we mean when we say we are finishing parts? Finishing a part is taking it from a rough, raw state that is right out of the SLA machine or off the CNC to a more defined, finalized state that is closer to the desired design.
When you are deciding how to finish your prototype you need to understand how your prototype will be used, and what level of finish is needed. Keep in mind that how you finish your model will help your audience understand your design. This is the visual representation of your idea.
Will it be a volume study, an appearance model, or something in between? Each of these have different characteristics and different levels of finish. A volume study may only require a limited amount of sanding to define the details or you may only put a coat of primer on it for a uniform look. An appearance model will require a greater degree of finish, as such, you will need to sand the entire part down, paying attention to detail areas and features. It may need to relay the intent for materials that could be used in the production model and represent the final product. With a higher level of fidelity prototype that’s needed, more refined and precise finishing will be required.
Depending on the material that you are working with and the final finish that you want to achieve, the steps in this process can be simple and quick, or can become very involved with multiple steps and materials. Keep in mind that not all these steps will apply in every case.
Once we understand the level of finish that’s needed, it’s time to get the process started! When first approaching how to finish a part, we look at what it is made from. Once the part comes off the CNC or RP machine, we need to evaluate it. How rough is this initial finish? The raw state will determine where to start with our finishing. The first step is intended to smooth out all the rough surfaces to get one, continuous surface. The surface will be rough but flat and the details will be defined.
When considering what grit sandpaper to start with, it’s important to always choose the highest grit that will do the job efficiently. Down the line after primer, this helps to avoid having to sand out large scratch marks from low grit paper. But how the part was made will influence the grit paper to start with.
Some rapid prototype parts made on an FDM machine will usually need 80 or 120 grit sandpaper to knock down some of the large steps related to that process. In contrast, a high-resolution SLA part may start with 180 or even up to 320 grit sandpaper because the layer heights are much lower, meaning the part needs less overall work before primer. CNC machined urethane tooling board parts can be run with tight tolerances depending on the machine. In this case, you could fall anywhere between 80-320 starting grit. When sanding on clear RP parts or cast clear urethane parts we typically wet sand up to a very high 1,000-2,000 before buffing or clear coating. Wet sanding helps to carry away the dust particles, so they don’t scratch the part and helps reduce clogging up the paper.
As we go through the sanding process, we move up to a higher grit paper than what we started with. Within this step, the intent is to work toward removing the heavier scratches from a lower grit paper. It’s important to pay attention to the details of the part such as radii, chamfers, and how the part surfaces blend together. When finished, the part should have a flat, uniform finish and the details are clearly defined.
Then, we continue to move up to a higher grit paper. This is to remove the finer scratches left from the previous sandpaper. It is amazing how easy it can be to sand off small details and how much time it will take to return those details to your part. If too many details are removed, it may be necessary to go back and start from scratch by re-running the part. This can cause lost time and add cost from overruns, neither of which is a good thing!
When the part is completely sanded and the surfaces and part details are nice and crisp, we then have two choices: continue to go up to higher grit sandpaper and repeat the process or move on to prime the part. This really depends on the final paint finish that is needed. If the final paint finish for the part has a texture, then it is time to move on to the primer step. If it has a high gloss finish, it may be best to continue the process of moving to a higher grit sand paper with each stage until all the scratches have been removed. If the scratches are not fully removed, you run the risk of them showing through your final paint.
Throughout the steps of sanding and finishing, many tools of the trade can help get us to the desired level of finish faster and with fewer missteps. So, what are the tools that can help get the job done well?
There are some tools that allow us to use sandpaper more efficiently. A sanding block is a very basic, but also a very important part of sanding. With any grit, the sandpaper gets wrapped around a stiff block. There are also some pre-made blocks by 3M, Norton, and Klingspor. A block could also be as simple as a piece of wood, metal, or rubber cut to whatever convenient size is needed. Using a block helps the flat areas that are being sanded stay flat. Without a block, it is easy to make grooves in the part where pressure is being applied unevenly across the paper. It also helps to highlight if your part is not flat because most materials will change to a different shade as it is getting sanded away. Once the entire surface is the same shade then it is properly and evenly sanded.
When smoothing out a part or scuffing it for paint, a fast way to take an even surface off is to use a sandblaster or soda blaster. These use different grits of media and air through a nozzle at high speeds to put pressure on the parts and wear down the surface. With a delicate part, an alternative is to use a very light media like sodium bicarbonate as it will evenly and lightly smooth the surface. With more aggressive steps or scratches to take out, there are medias like aluminum oxide which is similar to using 80 grit sandpaper.
Another great time saver is a power sander. We typically use a dual action pneumatic sander, but a random orbit electric sander works similarly. These machines have two axes that rotate at the same time like small circles rotating around a large one. This action helps to avoid swirl marks in your part and leave consistent grooves. DA sanders can help save time and energy if you have large flat areas as they are many times more efficient than a human hand alone.
We frequently use sanding pads or sponges that can be used to help finish out parts. These are a different grit of sandpaper that is attached to foam padding and are very useful around radii and to smooth out a more organic shaped part where paper typically tries to stay flat. They are also very useful on lightweight urethane tooling board where even higher grit sandpaper seems to leave scratches because of the softness of the foam. Scotch Brite pads are similar in function to this but are typically used to lightly sand or scuff plastics or urethanes before going into the primer stage to help it have better adhesion.
When working with machined steel or hardened parts, using a file or polishing stones helps to work up to the desired finish and not wear down your sandpaper as fast. When polishing a metal part, we will sand it up to a high enough grit and then use a cutting compound and a buffer to get a mirror shine. We typically use Meguiar’s mag polish on aluminum and stainless.
Sometimes the best tool to finish out a part is a scraper. This is usually a sharp piece of metal with or without a small burr that you drag across the part to sand away the surface. Sometimes these can be as simple as the edge of an X-Acto blade or something pre-made like a woodworker’s card scraper. It is especially useful when getting down into small grooves or getting rid of parting lines on cast urethane parts.
When there are damaged parts or parts that need to be glued together and seamlessly joined, we will often use autobody filler. The filler is a two-part catalyzed system that is mixed together and used to fill in gaps or chips. Once it dries, we sand it down to make it flush with the original surface. Another trick for small chips or pits in plastic parts is to fill it flush with baking soda and drip a small amount of thin superglue over the chip. It wicks in and hardens very quickly. This trick is especially useful for cast parts that are getting painted with bubbles or RP parts that chip out from the supports.
It may seem like a long, detailed process to even get to priming, but the time you spend upfront, will lead to better prototypes, and less re-work. It is better to refine the details now, then to have a poor paint job that leads you back to square one. Re-running, re-finishing, re-priming, and re-painting are never activities that help you in the long run.
Now as you enter the priming stage, it’s time to determine which primer to select. At Priority Designs, we use different types of primers for different finishes and parts. Priming application depends on the level of finish on parts and the material being used.
The most common primer is a catalyzed high build primer which allows for a buildup on parts to fill scratches, pinholes, and low spots. The more time spent on preparation, the less time that will be needed for sanding in the end. The primer can be reduced to thin it out for a smooth layout leaving the end result in faster prep time for paint.
When working with a foam, wood, or any material that absorbs, we use an extra high build polyester-based primer which is a catalyzed liquefied body filler. This will fill quickly and give you a base to work from. These extra high build primers, or just the typical high build primers, can be used on all of the materials we use, but depending on the material there could be an additional step involved. For instance, when working with metal, galvanized metal, aluminum, and fiberglass, we would also use a self-etching primer or an epoxy primer. These products are used to seal a part so it doesn’t rust or corrode. This will promote maximum adhesion for the priming stage and eliminate any peeling issues.
Each part will need multiple layers of primer. After applying 2 to 3 coats of primer, this will provide the buildup and material needed to sand the part, allowing you to finish everything out. Keep in mind, light multiple coats are best rather than one heavy coat. After the primer has dried, it’s time to sand the part again. At this stage, we don’t go below a 320 grit sandpaper, and may go even finer. The goal here is to remove scratches, not add in more!
When sanding out the part, it’s important to be careful of the details. We’re very close to completing the project and we don’t want to ruin it now! After sanding, we may need to prime the part again, and sand it out to an even finish. Depending on the part, this process will need to be repeated as many times as it takes to get to a smooth surface finish. More time invested in the early stages of sanding will usually equate to less finishing at the end.
The last step after you have sanded and primed your part will be to go over the entire part with an ultra-fine sanding sponge. The sanding sponge has a very fine finish and is used to catch any small scratches that you may have missed. The end result will give you a smooth, scratch-free finish for the part to go off to final paint for the ultimate finish.
Unfortunately, if you take a part off the machine and give it a coat of paint, you’ll likely be sorely disappointed by the results. The finishing process simply cannot be skipped. Having the right tools and going step by step through the process will ultimately get to the final prototype that looks just like your initial design. But it takes time (and patience, too!). After the part is properly sanded, finished and primed, it’s onto the paint booth for color, texture and all the bells and whistles of a finished project.
In the end, having a high-quality prototype that’s done well the first time can help move your project along smoothly, and on time. And I think that’s a good result we can all get behind.