What is the old-school way of smoothing a curved surface?

Question

I have heard that the old-school way of creating a flat surface is to use a hand plane, and that if you use the right tools, you may not even have to sand because the surface is so smooth. Can I use a flat hand plane to smooth an flatten a curved surface? Is there a tool specifically for it?

The surface in mind is a U shaped stool top, so there is only a single curve, though I have to handle both the inside and the outside.

I know I can just sand it, and that's what I plan to do if I don't have an alternative. Is this the easiest/classical way of handling such a surface?

Answer 1

This is a tangential point to your main question but has a bearing on my suggested solution:

I have heard that the old-school way of creating a flat surface is to use a hand plane, and that if you use the right tools, you may not even have to sand because the surface is so smooth.

While that can be true it does tend to get overstated by some planing enthusiasts. It was actually quite common in the past for even the surface left by a "smoother", or smoothing plane, to be further refined. Be it by scraping and/or with some abrasives (even before the advent of sandpaper).

Can I use a flat hand plane to smooth an flatten a curved surface?
The surface in mind is a U shaped stool top, so there is only a single curve

So I'm presuming this is dished, i.e. has a concave surface. Generally speaking a regular flat-soled plane can only be used on convex surfaces, not on concavities because the flat bottom will 'bridge' from toe to heel and prevent the cutting edge from contacting the wood.

There are a number of other edged tools that can be use for concave surfaces. In terms of smoothing the surface specifically (rather than creating it in the first place) the main one is the compass plane.

The all-metal version had a flexible steel sole whose curvature could be adjusted to match the surface you were working on:

Wooden versions are also an option and these can have soles that curve in one or both directions, the latter allowing work on compound curves (a surface dished both front-to-back as well as side-to-side).

Compass planes are specialised pieces of kit and can be quite expensive, but are of course purpose-made for the job and do it well and efficiently.

There is a simpler option, one much better suited if only a small amount of smoothing is needed and that is the curved card scraper. Card scrapers are widely sold in sets of three shapes, and this may be the ideal way to buy them as all three shapes will prove useful. For use on a hollow seat the best choice is the one that looks a little like a stylised comma:

All card scrapers can be used in either a pulling or pushing motion depending on user preference and the demands of the shape being worked.

After scraping you may end up with a surface you deem finish-ready, but this isn't always possible (due to vagaries in the wood as well as how sharp the scraper is) so don't feel any embarrassment if you need to do final smoothing using sandpaper.

Answer 2

There is a compass plane, a plane with bent or bendable sole.
Unfortunately I couldn't find an image with a proper license (sic) The radius on a compass plane is probably too big for a stool.

There is also the spokeshave with much shorter sole.
It can be concave or flat or convex.

A third solution is to mount the stool top on something that lets it rotate and then plane with an ordinary plane lain at side.

The fine touch you can do with a card scraper.

Answer 3

Was also going to suggest mini block planes as well for the outside of you "U". Their size lends to detail and you can easily shape the blades to be soothing blades which prevent the edges from digging in (More used for flat surfaces but if you already own these tools it could work.)

While looking at pictures I also found references to shaping planes which can be used on both the inside and outside of your "U". In the following picture the tools are upside down so you can see the cutting side/face.

^{Image from Leevalley Tools}

Side note

The other answers do a good job of mentioning tools for working curved surfaces. I would like to make a small addition since you were talking about old-school methods.

I realize that you are looking to refine in this particular case but uf your stock is large and you want to remove larger portions quickly then you could use a draw knife ( also known as a pull knife ) before you get to the smoothing tools.

^{Image source: Wikipedia}

Answer 4

The tool for this is called a scorp. There are small ones for carving out wooden spoons, and large ones for chair seats. They are like a drawknife, but curved.

Here's an example:

That said, unless you are making a lot of these, and willing to invest a fair amount of effort, it might not be worth it to buy a scorp. A curved hand scraper (just a 3 x 8 or so sheet of steel with a curved, honed edge) will get the job done.

Answer 5

Generally speaking you would use a spokeshave which is like a plane with no sole.

You can then refine that edge with a card scraper to make it smooth.

Answer 6

I highly recommend a Stanley 113 - compass plane. The base of the plane is adjustable allowing for you to plane different angles. Remember if you are doing something like a rocker you can use a jack plane on the convexed portion and the compass plane on the concaved.