I have had people within the floor sanding industry with 15+ years experience tell me that the correct way to sand a parquet herringbone floor is to go diagonally across the floor. That means with the grain of some blocks, and against the grain of others. I believe this is incorrect for the most part and here is why:
Wood is essentially long fibres densely packed together. All the strength travels in one direction and not the other. By sanding with the grain you are sanding with the fibres of the wood and not displacing them. This means there’s less abrasive scratching. By sanding against the grain of the wood you are tearing these fibres and stripping away the wood much faster, this makes for a much faster and aggressive sanding process.
If the grain of the wood is all running in one direction, it would be ideal to sand in the same direction every time(with the grain). Unfortunately, as you will learn, even this presents problems. Sanding with the grain is best because its less aggressive and you have less scratching, making for a smoother, better looking finish. When sanding herringbone parquet diagonally you are sanding with the grain on half the blocks and against the grain on the other half. This means that the blocks which are perpendicular to your sanding machine are going to be sanded down deeper than the blocks on which you are sanding with the grain. This alone can lead to a lumpy uneven parquet floor.
To add to this, sanding diagonally will inevitably mean that, for small periods of time, the drum will passover (and have all of its weight on) one parquet block. Each individual block in a parquet floor varies in density from one block to the next and can even vary within the block from one side to the other. So when the sander passes over a block that is much less dense than the rest of the floor and the drum is sanding that one block alone, it will sand down much further than the rest of the floor. (When I say much, I’m talking millimetres, but throughout this process, it all adds up to a lumpy floor)
Now before I carry on I should say. This variation in density means that the floor will never be perfectly flat. Getting it as flat and smooth as possible is our goal.
By sanding straight and across a parquet floor it deals with both of these
problems. First of all it means that all blocks are being sanded with an equal degree of aggressiveness. But the main benefit of this method is that the drum will never be sat on one block alone. It uses the varying densities of all the blocks to regulate the pressure on each other block .
Next you should switch direction from straight to across. This helps to eliminate any imperfections caused by the sander. This is especially true of people using hire tools as they tend to leave chatter marks across the floor and even cut in on one side leaving a ridge in the wood. So your process should be straight on 36grit, across on 50g, straight on 80g, across on 100g and finally straight on 120g.
There are occasions when the rules need to be broken. Sometimes it is not possible to change the direction by 90 degrees, purely because of space restraints. For example, when sanding parquet in a narrow hallway, you must find another way. There are many different solutions (and infact you can tailor your floor sanding method to any floor depending on the wood, pattern, size, damage and time restraint). A good general idea is to just do the alternate sanding 22.5 degrees off the straight sanding (you dont need a protractor just judge it, half way between diagonal and straight :)), this way you are not going directly with or against any grain and you wont be going in the same direction you previously sanded. Its not perfect because sanding at that angle will be more less aggressive on the blocks that are almost with the direction of the sanding and more aggressive on the blocks that are almost totally across the direction of the sanding.
Another time you may want to consider breaking the rules is if you anticipate that the linear sanding scratches from the belt/drum sander will not come out with the buffing process, or (as will probably be the case with a lot of DIYers) that there will be no buffing process. An example of this is sanding pitch pine parquet. The sap (moisture) content of the floor makes it incredibly difficult to get the scratches out because the abrasives get clogged up and ineffective. If this is the case, you should have noticed it early on. In this case I would definitely recommend doing the final sand diagonally so that if the scratches don’t come out after buffing (or you don’t buff at all, DIYers ;)), they’ll be relatively unnoticable.