If I judged this from number of opinions in forums, how to sand is one of the more controversial issues. If I judge it from reading books from the finishing experts, they all seem to agree quite well. So I guess this is an issue which is many times about principles rather than about facts. However, tjhrough trial and error, I have learned that if you want to use high gloss paint, you have to be a lot more careful than if not. So let us first set up a number of issues to remember:
- You must sand with something so coarse that the finish (we here talk about waterborne paint) sticks after being applied.
- You must sand with something so fine that the finish looks even and has a chance to reach the level of gloss you want
- You must sand in such a way that you do not unneccesarily pass through the paint layer down to primer or wood
- You must sand in such a way that you remove the uneven surfaces that would prevent the wanted end result
The above actually results in two sets of rules which both says that there is a maximum and minimum coarseness to your sanding, and a maximum and minimum amount of removed material as well.
This involves two traps:
- The trap of different international standards to define how coarse the sandpaper is. This confuses the discussion about coarseness by referring to different coarseness when using the same numbers
- The trap of balance between spending too much time to achieve the result, and taking too big risk to destroy the result.
We need find a way to discuss the second issue without spending too much time on the different coarseness systems. This we can do by assuming we have a quite rough piece of wood to start with. This becomes easier to discuss since the american and the european systems are very similar for the coarser sandpapers, while starting to differ in the medium grades until becoming totally different at the finer grades.
- The first principle is to use the coarsest sandpaper that will remove the material you aim to remove, without risking to go through the layer.
A base principle is that whatever grit sandpaper you use, the easiest way to remove the scratch marks from the sandpaper is to use the next grit size. Thus, if you start with a too coarse sandpaper, you will have to remove too much material to get rid of the sandpaper you started with. If we talk about before you have applied the primer, you should probably not start with something finer than P120 (european standard, corresponds roughly to 120 american standard, it is quite easy for these coarser papers). Then use P150 (150 american), P180 (180 American) and end with P220 (220 American). We have now achieved two things:
- We have reached the smoothness needed for waterborne finish. With another finish, we would probably have stayed at P180
- We have reached the end of the scale where the american and european numbers for sandpaper roughly agrees with each other.
We have also done it in the most work efficient way possible. By stepping up one grit size at a time, we have targeted first to get an even surface, then to successfully remove the grit marks caused by the previously used sandpaper. This method wastes sandpaper. But jumping from 120 to 220 would make the job to remove the scratches from the 120 sandpaper a lot tougher. If you can use a machine on the surface, skip some papers. If the wood is soft, skip some papers. But if you are working on hardwood, and by hand, you probably should follow the steps as defined.
Whatever you have behind the sandpaper affects the end result. If you have a very hard piece of wood holding the sandpaper, you are likely to get into unnecessary trouble. Such a hard piece will not yield at all. Even if you start on a very flat surface, you will likely get something sticking to the sandpaper. If that happens when the sandpaper is held against something which does not yield, you will scratch the surface, or sandpaper it uneven. Thus, use something that is suitable:
- Cork is quite good. Cork on top of hardwood is also good. It will keep its form, but will yield a bit. It is thus a very good alround holder for sandpaper
- Depending on what to sandpaper, holding it in hand or fingers is also good. Specially when there is a unflat surface, or a small top, or close to a corner or edge, holding it in hand is a good choice.
- Finally, the hard piece of wood that in general is a bit dangerous to use, is perfect as a holder when you objective is to only remove small extensions on the surface. Achieving a flat surface is best done with a scraper or similar. But removing isolated high spots, like dust in paint can be done with sandpaper. In that case, hard wood bewhind the sandpaper is a good choice.
For a power sander, like an orbit sander, the same rules apply. If you use a too hard pad behind the sandpaper, it may start building a ridge which destroys your result. If you use a too soft pad, you risk the sander bites into the edge too hard.
A good tip is to start working close to the edges and corners. The risk of going through is the biggest there, so by starting there, you will either have sanded them to desired result, or reached a level where you know you can do no more without going through. When corenrs and edges are done, you can move into the rest of the surface. If you use an orbit sander, a good tip is to lean it slightly in such a way that the side where the round plate bites most into the surface is the side furthest away from the corner or edge.
After the first layer of paint has been applied, the next sanding to do depends on the result. If you did get runs, you should cut rather than sandpaper, in order to first get a flat surface. If you did get orange peel, you might need to remove a lot of material. However, you do not want to sandpaper through the paint, and since you will need to follow up the first sanding with removing the scratches using finer sandpaper, I would very seldom consider to use anything coarser than P240 (240 American scale) for the first sanding after painting. I would if this was the last layer continue with P400 (320 American scale), and P1000 (500 American scale). I would nowadays use water together with the sandpaper. It makes a huge difference, although it will feel uncomfortable on MDF. If this was the last paint layer, I might also start with P1000. Or if it looked really good, start with a rubbing compound and then follow up with a polish for finish. The major things to think of here are:
- What strategy is used for sandpaper (or cutting) after the first paint depends on how the first paint went, and if this is the last layer
- For these sandpapers we see that the American and the European system differs a lot. Instead of more or less matching numbers, we now start getting a double of the numbers (500 American equals P1000, 400 American equals P800). The difference is clear from P320 and upwards. For example, P400 and 320 American is similar coarseness, but the numbers are far from each other
- For the last layer, sandpaper is not the solution unless there is a need. For the last layer, the target is to polish the paint.
- Waterborne paint does not enforce you to sandpaper between layers in order to assure the paint does not come on in separat distinguishable layers. Thus, unlike for some other paints, you do not need to use a coarse sandpaper to assure the paint stick to the previous layer.
For sanding on the paint, use wet sandpaper and use water. The water acts as a lubricant and makes the sanding much easier to succeed with.
There is a number of die hard power tool fans out there. I believe most people that start to work with wood today start with a belief in power tools and want to get good enough power tools to get fantastic results. There is also a number of those hand tool entusiasts out there. Some of them prefer hand tools, some of them believe hand tools are a good complement to power tools. Myself, I started out in the power tool category, and has been forced to convert into the hand tool complement category. I now believe it is needed and easier to do some last adjustments to get the wood project to look right. Those adjustments are often best made by hand tools. Thus, although I own a good set of power sanders (belt sanders, orbit sanders, delta sanders, finger sanders) of good quality (mostly Festool). I find myself more and more often to use a hand plane.
For paint, it is slightly easier, you are not expected to hand plane the paint. But if you have runs, do not try to use a sandpaper to make them even, instead start with some kind of cutting tool. After the surface has become fairly even, then use sandpaper.
The thinner the paint, the bigger the risk to sand through the paint. Thus, a power tool is a very good solution when only polishing in the end (thick paint and not much sanding action). If the paint has gone on well, and you are using a fairly fine paper, power tool might be a good choice also from the beginning. However, when dealing with runs or orange peel, you might save yourself some work by sanding by hand instead of using a power tool.