Sanding hidden surfaces

[John Strong]

Hi everyone,

This may be a slightly stupid question, but to what extent (if at all) do people sand their hidden surfaces. Are there any benefits to sanding them?

On another note. Surfaces out of the planner I've been hitting first with 120 then 180 with an ROS. How high do people normally go/ spacing between the grits?

Best,
John

[Mel Fulks]

1, no benefit ,unless you think something might be prone to splitting from temp changes. Summer cottage used on
week ends, with no AC or heat when unoccupied . Then some sealing might prevent damage, the sealer would probably
need a little sanding ,you don't want rough surface any where on something good.

2. I would not skip any grits. 120 then 150 is probably all you really need. But don't skip

[Tom Bender]

As a hobbyist who enjoys the road I sand early and often for a few reasons.

It prevents splinters

Layout on sanded surfaces is more accurate and easier to see

Parts register better against fences etc

Finishes turn out better if applied early

Defects show up sooner

Defects are easier to correct

It's more pleasant to work with nice surfaces

[Kevin Jenness]

Sand hidden surfaces if it makes you feel good. The cost/benefit ratio may be different for pro vs hobbyist. Sealer will stick to planed surfaces. Sliding surfaces may run smoother if sanded.

You can skip grits but you have to sand a bit longer at each step. Don't jump more than 50% above the previous grit #. A typical schedule might be 80 (for severe tearout), 120, 180, 220 or 100, 150, 220. I usually stop at 180 for film finishes, 220 or 320 for oil finishes and stain. Stains and gloss paints will always show your sanding flaws. A strong raking light will help the process.

[Jim Becker]

Hidden support pieces and secondary wood only needs sanding to the extent that the surface is consistent for accurate joinery and gluing. It doesn't need to be pretty. I generally wouldn't go beyond that first hit with 80 grit which is my lowest starting point in most cases.

[Ed Aumiller]

When making furniture I sand all hidden surfaces with 150. Do not sell anything, just do it for my enjoyment. If putting a lot of work into something, quality matters a lot...

Also, it is best to put some finish on both sides of wood to prevent / reduce changes due to moisture.

[Mel Fulks]

Nothing wrong with more sanding. Most of us are making stuff as a hobby,I mean nothing pejorative there , sure some are
brain surgeons ! But experts in early 17th or 18th century work will admire and point out "economy of labor" and "expert fret-work done only with saw ...not sandpaper", as well as "fine
carving", "eye for proportion", etc.

[Kevin Jenness]

I don't mean to disparage work finished equally everywhere, just to point out that "quality" means different things to different people and at different times. Many masterpieces of furnituremaking from the days of handwork have surfaces straight from the pitsaw or scrub plane where the clients were not likely to look. With the technology used today it is just as easy to have both faces of a board planed parallel as not and a whole lot faster to get from there to a "finished" surface than back in the day.

[Mark e Kessler]

Depends on how “unseen” For fine furniture I sand hidden surfaces to 120 ROS then follow up with hand sanding 120 to remove the ROS marks. On the bottom of a table, I consider it the same as the top as even though it is “unseen” it is very accessible, for example I would go to 180 -220. I only do oil finishes now, in any case if it can ever be seen with an eye “hidden” or not I never ever leave the “swirl” marks from an ROS this is how I was taught... for whatever reason I have never forgotten what an instructor told me which was it’s easy to make the main surfaces of a piece to look good but true craftsmanship goes beyond the obvious...

[Lee Schierer]

Most of my projects are made from s4s hardwood. All exposed surfaces get sanded to 220. I do this to insure any finger prints, glue smears, cat footprints and dust are loosened and removed before finishing.

If an assembly will be too tight to sand or finish when assembled they get sanded to 220 and finished as pieces. I mask off any joint areas with painter's tape.

[John Strong]

Thanks for all the responses, hearing everyones perspective was very helpful. The inside of the dresser carcass I'm working on is going to be very quickly hit with with 120 (which happens to be the lowest grit that came in my multi pack) for some personal satisfaction.

[Jim Becker]

Thanks for all the responses, hearing everyones perspective was very helpful. The inside of the dresser carcass I'm working on is going to be very quickly hit with with 120 (which happens to be the lowest grit that came in my multi pack) for some personal satisfaction.

That's actually a good thing for a dresser because you want the inside surfaces smooth so that any clothing that comes in contact with them when someone invariably overstuffs a drawer will not get caught on rough surfaces. You can't see those surfaces, in general, but there is a good functional reason to at least smooth them out. 120 is very reasonable for that. And as has been mentioned. clean components can be helpful for joinery accuracy, too.

[Ernie Hobbs]

I think my answer is consistent with most here. If it is truly hidden, then it doesn't matter. I typically only sand if I have a pen mark that I want to remove or I need to clean up a stain that doesn't belong there. But I guess it it can't hurt, as long as you don't screw up your joinery.

My sanding progression: plane (hand or planer, depending on what I'm doing)> 100 grit > 220 grit > maybe 320 if I want it to be really slick.