From First Sand to Final Coat: How to Get a Professional Finish on Layered Wood Sheets
Most finish issues with plywood stem not from peeling paint but from the wrong grade purchase, the first sand, and wrong grade of that. Get these three right, and the rest is comparatively simple and mechanical. Get them wrong, and you are stripping and recharging half the sheetrock by the time you meant to be hanging the last sheet yesterday.
This is the secret the finishing guide at the paint counter, where you were shown a ten-step process and a picture of a glorious lacquered table, did not share with you. The steps are important, as is the order, but so are the decisions behind them if you are not on a well-laid-out schedule.
Match the grade to the work before buying anything
Most grades are identified by a letter on the face and the back. This letter tells you how much work you will have to do to get that sheet ready for painting. On the upper end, A-grade faces are smooth, with no patches. Meanwhile, D-grade sheets will have knots, patches, and voids that you will have to fill and sand for three cycles before you are done. The mistake most people make is to buy the slightly cheaper construction-grade CDX, only to discover that they have to fill and sand three times more than they expected.
This can make them invest in a much larger wallet, as the time spent filling and sanding, plus the sandpaper, filler, and labor costs can offset the initial purchase price. It becomes a much better deal to pay for a grade B or a hardwood face (birch, maple, or oak) sheet if the face will be seen. The birch, maple, or oak faces have good quality veneers with no patches or voids and, if desired, can be stained or finished with clear lacquer with minimal sanding.
On the other hand, CDX is totally fine for use as a backing if the face will not be seen. This is why it is worth your while to buy from a proper supplier. It is worth going to the actual supplier, such as Plywood Supplies Perth, to ask about the grain and voids in the face before buying. This lets you know what you are getting and can give you advice on what grade to choose.
Sand right, and in the right order, and you will have problems later.
The common short-cut which results in visible marks weeks later is to skip grades when sanding. The right order is 80, then 120, and then 180 or 220. Each step is meant to remove the lighter scratches left by the previous sanding block. Going directly from 80 to 220 will leave fine swirling marks that are mostly invisible on raw wood but stand out like neon lights when the stain or clear coat has dried.
You must always sand with the grain. Skipping across the grain leaves scratches in a direction that the wood grain cannot disguise, and these will be much more visible on the finished product. Additionally, if you are sanding something with a paper-thin veneer face, you can sand through the face veneer and into the next layer. These cannot be sanded out, as you can only sand with the grain. One pass per grade is sufficient if you are being careful; there is no reward for going over 220-grit sandpaper and closing the pores of the wood.
Fill all the patches, and fill them correctly, or they will haunt you later
Lower-grade sheets tend to have patches and knots that need to be filled. Your instinct should be to fill those patches, sand them flat, and move on. However, you will find, weeks later, that you have depressions where the patches were. This is because the filler shrinks as it dries, and even a single coat will leave a noticeable mark after it has fully cured.
To avoid this, you should proactively treat all filled patches as needing two coats. Fill the patch, allow it to cure to a hard state, and then apply a second coat, which you will then sand flat. This will take an extra fifteen minutes or so, but it will ensure that the depression is completely gone. If you have a large patch, some finishers like to use two-part epoxy putty instead of wood filler, as it does not shrink as dramatically and is much tougher if the panel is going to be exposed to the weather.
Seal the sheet before attempting to paint or stain, without exception
Unfinished sheets will soak up whatever you put on them unevenly. This can lead to patchy results that you cannot fix by simply applying an extra thin or thick coat on the affected spot. The veneer face, the glue lines between the plies, and the exposed wood on the edges all absorb stain and paint at different rates and will stand out no matter how many extra coats you apply.
Applying a sealer before attempting to paint or stain prevents this problem from occurring in the first place. It could be a sanding sealer meant for the job at hand, or it could be a thinned first coat of top coat or a shellac-based primer, depending on what you are using. Whatever method you choose, you want to ensure that the absorption is even so that the color coat will turn out smooth and even.
Regardless of the method, this step should drastically reduce the number of coats you will need to apply. Two finish coats are usually sufficient if you have a properly sealed and filled sheet, while three were going to be necessary if you had not taken this step. This is a significant difference, especially if you are working with larger sheets of plywood.
It is standard for industry coating trainers to say that up to 80% of all coating failures stem from inadequate surface preparation. It is also common for this step to be the largest single cause of these problems. It is, therefore, worth the extra time it takes to complete this step.
Block the tannins if you are going to use water-based paint on pine, oak, or other tannin-rich wood-faced sheets
Water-based paint cannot deal with tannins, which will bleed out through it in blobs that look like brown stains. Shellac is your friend here; it seals the tannins and works well on knots and other resin-heavy places that need to be blocked before paint can stick to them. The standard procedure is to apply shellac primer, sand the sheet flat, spot-prime any problem spots, and apply two finish coats on top. It is extra work and extra sanding of your prime coat, but it saves you the hassle of having to treat all the resin-heavy spots specially.
Treat the edges as though they matter, which they do
A good face with raw edges always looks like a beginner’s project no matter how good the face is. The reason is the exposed plies on the edge – they are some of the most common giveaways that a project was not made by a professional. Fortunately, this unsightly detail is also among the easiest to fix.
For straight edges, iron-on edge banding is an affordable, quick, and reliable way to finish the edges while giving them a clean, straight look that sand or paint will adhere to consistently. For edges that are not straight, you can either use edge banding or fill the exposed plies with wood filler and sand them flat. Either way, you should not overlook the edges and plan to spend the time needed to finish them properly. You should budget for this when you are planning the job, not when you are dry-fitting the sheets for the first time.
Pick your finish based on the environment the sheet will be in
Finishes on hardwood sheets are classic – they can be finished with polyurethane (oil-based for amber tones, water-borne for less yellowing), or, for a more natural appearance, a wipe of either Danish or tung oil to give the sheet a hand-rubbed look. If you are in doubt about the effects of any particular finish, or if you want a maintenance-free option that still makes the wood grain stand out, you should consider paint or high-build finishes.
If you are doing exterior or marine plywoods, you will need to pick your finish with more care. Interior finishes blacken and yellow under the influence of ultraviolet light, so you have to pick either a UV-filtering spar varnish or a full exterior paint system to finish your work. Marine plywoods have fully waterproof cores, but the exposed edges still need to be sealed before being exposed to any moisture. Water will run down the edges of a marine-grade sheet along the exposed end grain, soaking it to the core regardless of the grade.
If you are using a water-based finish, you need to consider the effects of grain raising. This phenomenon sees the grain of the wood raise when wet, creating tiny bumps that rough up the surface. To prevent this from happening, you can wet the sheet with a damp cloth, wait for it to dry, and then sand it lightly. This will remove most of the bumps without leaving you with a heavily scratched surface that needs extra sanding.
Plan the materials, not just the steps
The single most important consideration when budgeting for this project is the labor. The difference it makes to pick the right grade and the right primer, or the right filler for defects can be measured in hours saved. It is essential to consider these potential hours when considering your options.
A failed finish not only wastes the paint but also the time and effort that went into preparing the surface for it. You will have to sand everything down to the raw wood, prime it again, and apply an additional top coat. This is why these steps should be considered when you are budgeting for your materials. In many ways, these are much more significant expenses than the materials themselves, so it is best to think of them as separate items. After all, a second round of sanding and priming is much less expensive than buying an entirely new sheet of plywood because the first one was ruined due to a failed finish.
You will not get that professional finish until you consider all the decisions that must be made before even touching the paintbrush to the sheet. The right grade, the right sanding steps, and the right primer are all critical considerations that will determine how easy (or hard) the finishing process will be.