When I purchase wood from the local carpenter a lot of boards are 1-2mm off which results in gaps in my furniture which I usually just fill with silicon. I don't have a sanding machine with a length equal to the board (say 1.5m at max). My local carpenter insists that errors in the millimeter range are unavoidable.
Is it even possible to do this accurately with handheld tools?
By hand I'm afraid there's no easy way to remove excess material from the edges of chipboard (particleboard for those in the US). In general the idea is to saw it to the exact size you need because it's so difficult to alter afterwards.
Planing
You can plane the edges of chipboard with one or more types of hand plane, but the glue in chipboard is extremely hard and very abrasive to cutting edges so with most plane irons you have to stop frequently to resharpen, making the process very slow and difficult to rely on even to shave off a small amount every now and then. And 2mm is not a small amount when it comes to chipboard, even with the thinner stuff.
Belt sander
A hand-held belt sander with a coarse belt fitted would be a much better way to remove this much from the edges, particularly with the 30mm, although I realise this may not be an option for you for any number of reasons. In addition to the (possibly high) cost of the sander and the (possibly high) ongoing cost of replacement belts the large amounts of dust produced could be a big issue for you*.
Power saws
It's not a standard use but it is theoretically possible to nibble away material using a circular saw (and for thinner material a jigsaw) running the baseplate along a straightedge set up an appropriate distance from the edge. I wouldn't want to recommend this though unless you already have one so can easily test it out at no cost.
Surform
I'm not sure how widely available Surform-like tools are in other markets but I've read they can be used for gross material removal from chipboard edges. How accurate you can be with one I don't know but I presume a lot of that is down to the skill of the user. Despite how crude the tool looks it could be used with finesse, just like with a conventional rasp.
My local carpenter insists that errors in the millimeter range are unavoidable.
I think it's important to state that they may be unavoidable for him, but that far greater accuracy than this is possible.
Sub-millimetre accuracy is perfectly feasible using good power tools set up and used correctly, with accurate initial marking up as needed. Commercial cabinetry could easily have accuracies in the 0.2mm range to you an idea of the level of accuracy possible, although I believe +/-0.5mm might be fairly typical.
Now that said, I'd prefer better but accuracy of 1mm either way might be acceptable to me for chipboard furniture depending on the application. 2mm would not, especially if short!
*Even with dust extraction some to a lot of dust can still be released (depends on model and the power of the vacuum attached). The dust from chipboard needs to be considered hazardous and breathing protection should be considered a must.
