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Bubbles or dust?
In continuation to my comments with some of the video lessons, I am now posting a picture of my problem
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Just for useful context for anyone else coming across this post I will put my original points from our conversation within the course chat below:
Ok, this could be a few different things, if you are able to email or post in the forum a picture of the surface then I should be able to advise further but here are an initial few possibilities:
1. Solvent entrapment: if you overcoat too soon then you can risk trapping solvents in the surface and causing this. This often looks like VERY tiny, microscopic bubbles and sometimes you can’t even see them until you begin sanding back. It doesn’t sound like this is what you have and 3 hours between coats should be enough time to not cause this.
2. Air bubbles: as I mention in this lesson, this is actually highly unlikely, bubbles in the varnish or induced by the roller will usually release within a few minutes of application. If they aren’t there initially and the wood is already sealed (and therefore off gassing is also eliminated) there is no way that bubbles can later form as the varnish dries.
A remote possibility in which you could induce these in your scenario… as you are applying “Wet on Wet” this can occasionally have the effect of lifting the first coat as the second is applied. It is the reason that coats 2 and 3 don’t usually look as good when using this practice. Sometimes you will see “stringing” of coat 1 as the coat 2 solvents lift it a little. There is an outside chance that in doing this you could induce a bubble in the sticky, lifted coat 1 varnish. This would not properly release due to the viscosity of the setting up varnish. HOWEVER, if you were seeing this happen, the bubble would be there immediately and would not appear over time.
3. Dust: the effect of dust landing in the surface can look deceptively like a bubble, especially if you begin sanding. The varnish clinging on to the dust and flowing around it forms a little crater that makes many people think it is a bubble.
As you are saying that these appear some time after application I am almost certain that dust is your problem here. If you want to eliminate the above possibilities 1 and 2, flat back your coat 3 and proceed with coats 4, 5 and 6 but leave a little longer between them, perhaps 24hrs. There is no way you will get solvent entrapment after 24 hours or stringing of the coat 4 varnish. If you have no bubbles present immediately after application but you are seeing this effect re appear in the hours following application then it is certainly dust.
Now, having seen the picture I believe this is a dust issue. I think the bumps you have that are smooth are ones where dust has settled in coats 1 and 2 and you have then varnished over it. The ones that look more crater like are from the last coat applied. I think the reason this looks worse in coats 2 and 3 is simply the cumulative effect of three coats worth of dust settling.
To overcome this, remember all of the points taught in module 7 about super clean workspace setup and application. Consider everything; are there people walking about in the room above you after you have applied? Are you turning the lights off when you leave causing once statically charged dust to now fall from the lights? Also consider dust or contaminants in your varnish and on your roller. Are you straining your varnish? Are your rollers AND trays clean? All of these can add a source of dust.
For this project in particular I would set up a dust sheet (using the 3M sheeting I use in the course) just above your board that forms a canopy over it. It can be suspended above the board, it doesn't need to have sides that extend down to the floor for a full tent but it will form a catch net for any dust within the room that is settling post application. Set it up as close to the board as possible whilst still allowing yourself enough room to properly work. Not only will it catch dust from above but its static nature will actively attract dust within the air rather than your sticky varnish. I think this will help you see a big difference.
If you want to do some experiments before risking application. Prep and cleanup your surface as you have been for application but don't apply anything. Come back after your 3 hour window as you have been and give the surface a wipe with a clean finger, can you now feel tiny dust particles settled on there? If so, this is what has been landing in your finish previously. OR, set up your canopy as I recommend, and after the same time window, give this a wipe to see what has settled on it. This is also what would have previously been landing in your finish.
Give those things a try and let me know how your results look.
This is the sort of setup I mean: https://danleeboatbuilding.co.uk/courses/varnishing-course-10-day-yacht-varnisher/lesson/module-7-lesson-5-clean-setup-for-varnishing-small-parts/
I am doing some small parts today and thought it was a good opportunity to make a similar setup and show the process. I think it will help to solve your problems.
Thanks, Dan. That is a nice setup and I have to think about how to do this in a bigger scale. I sanded the board yesterday and I will apply further coats over the weekend if I manage a tent in time. I will post an update in the forum and I hope the result is better next time!