Spraying Clear Coat and Dealing with Pinholes – Part 1

In a previous post, we covered preparation steps for clear coating laminate parts. In this post we are going to cover the clear coating application process and ways to help ensure a prime finish. There are many ways to achieve a mirror finish for fabricated parts. The exact process used may vary depending on the part type and the desired finish. We’ll also present various examples of why pinholes could be plaguing a given part and some best practices on how to eliminate them from the clear coating application process. There is a high amount of fine detail that goes into clear coating a part. These fine details in prep, selection, and application can make the difference in hours of extra sanding or kicking back to enjoy a clear coated carbon fiber work of art. Use Prepping Your Parts for Clear Coat for reference, this quick read article guides through ~80% of a clear coating application. Prep is Key!

Wax & Residual Release Agents

These types of unforeseen Foreign Object Debris (FOD) from the release agent may be the cause of clear coat application issues. Even with a quality wet-out laminate and light sanding from prep, residual silicone or wax from the mold release used on the mold may linger and lead to bonding issues with the clear coat to the resin. Most often this will lead to more than just pinholes.

Best Practice: Use a silicone and wax remover, degreaser, or IPA with a rag or lightly applied with a drenched scotch-brite pad before ever sanding the part. The goal here is to get everything off the part that may have been associated with its cure. Silicone is simply EVIL in terms of clear coating. Keep ANYTHING containing Silicone AWAY from the laminates being clear coated and the process of application.

Exposed Fibers

Another common cause of pinholes in clear application is exposed fibers on the surface of the laminate. Exposed Fibers may likely stem from an array of possible errors in resin introduction to the part during infusions or from bagging errors in prepregs or traditional layups. Traits of these types of errors are a rough mold side finish or pinhole type indentions in a part. The resin introduction, or lack thereof, may lead to pinholes in the clear coat application as shown in the right example of exposed fibers.

Exposed fibers may also be found if parts are over sanded during the prep process. Instead of a white or light gray film while sanding, there is black carbon dust evident during the sanding or cleaning process. In the case of exposed fibers, a clear coat will not stick to these areas, causing a pinhole or pitting issues upon application.

Best Practice: Resin acts as a “primer” for top coating and clear coating applications, effectively an “adhesion promoter”. The prepped resin surface is what the clear coat bonds to, not the carbon fibers. If the part surface is over sanded or inadequately wet out, place an additional resin layer(s) over the part. This may take more than a single layer depending on the part’s surface quality. If the part will be exposed to UV light degradation, this is a great time to apply a UV inhibiting resin or topcoat such as Duratec Sunshield. Thicker viscosity types of resin may be better in application as they will fill voids easier and apply thicker than other resin choices. For more on UV protection in laminates, see “Why Epoxies May Yellow & How to Protect Them from UV Exposure”. Even if there is no evidence of exposed fibers, a resin top coating such as Duratec Sunshield will promote clear coat adhesion and aid in pore filling imperfections in preparation for a high gloss automotive type clear coat finish.

In a perfect world, with all residual silicone removed and a completely wet-out and prepared part surface, application for clear is made simpler. But still not necessarily easy by any means. Sure, seasoned professionals may breeze through without any issue, but for most DIY composite fabricators, it can be trial and error, which is EXPENSIVE! Clear Prep for a clear coat effectively lies in knowing what details are important versus what can just be sanded out.

Sanding

Knowing how to effectively sand a part is critical in achieving a properly prepared part for a clear coat. Sanding is important because it gives the clear coat something to bond to on the resin’s surface. Clear coat will not bond well to an already shiny, possibly hydrophobic surface with high surface tension. The correct surface tension must be achieved on the part’s surface for proper adhesion of the clear coat by removing all shine from the part. Sanding, as part of the clear coat prep process, helps to lower surface tension, effectively helping the clear coat grab to the part surface just as prepping for secondary bonding practices.

When sanding it is important to achieve uniformity across the entire surface to be clear coated while not exposing fibers. Even after a resin coating, High spots may be evident as areas that still exhibit a flash or shine upon the area being sanded. All high spots need to be removed before clear coat application. The surface needs to be as level, this is usually achieved once the part is completely dull and flat with no tiny minuscule shine coming from the surface of the part.

Grit Size: There are various grit sizes that will be used effectively in different steps of the process, manufacturers of topcoats and clear coats generally call out specifically what grit range works best for a specific product. Note: Most grits can be used for multiple applications. One idea is to go ahead and order a variety packs of sandpapers ranging from ~180 – 600 Grit for clear coating prep and application. Additional higher grits running from 1000 up to 5000 Grit are generally used for polishing of clear coated parts. It is also a good idea to have a range of scotch brite pads, as they are great cleaning and prep tools and are easily applied with IPA or acetone or other cleaning solvent as needed.

Dry Vs Wet Sanding: Dry sanding is often performed when preparing a part for an additional resin layer, top-coating, or clear coating application. Wet sanding is for after clear coating application, as part of the process for bringing the part to a polished shine. Wet sanding helps keep the part surface cool and aids in removing residual dust from the sanded area. Wet sanding is often performed with a simple mixed solution of water and a drop of soap. Soapy water is commonly used to “stretch” the water, lowering its surface tension, making the water more effective for wet sanding.

Orbital Sanders: For larger parts, sanding is aided heavily by orbital sanders, paired with a softening interface pad and needed grit sandpaper being most effective. There are multiple sanding steps associated with achieving a mirror finish clear coat application. Orbital sanders can be used but with caution, especially with heavier abrasive grits. In addition to sanding large surfaces, orbitals are great tools to use for wet sanding clear coated surfaces as part of buffing and polishing processes. Use of Orbital sanders is highly dependent on the size or complexity of the part surface. For hard-to-reach areas, it is advised to use hand sanding techniques to flatten surfaces as needed.

Guide coats: Used to ensure an even surface is achieved before the application of a clear coat. It is one of the most effective tools for sanding and prep operations as it takes the guesswork out of possibly uneven sanding, visually preventing sanding too deep yet ensuring smooth surfaces throughout both prep and polishing processes. Guide coats are used widely across the auto industry as a standard in achieving the highest quality in application. It is as simple as a thin layer of sprayed paint. It helps to ensure clear coat applications are smooth! Guide coats can also be applied in all sanding steps from flat dull surfaces or in wet sanding operations when certain areas may be less apparent to the eye.

In Part 2 next week we’ll get to the actual act of clear coating & the best practices for insuring a stellar finish.