It’s just about impossible to cut two perfectly straight lines but you can cut two curves that exactly fit together. Overlap the two sheets you want to join. Temporally tape them down to your cutting table except the edge you are going to cut. Use a straight edge to flatten the cut edges together and use a very sharp knife. I prefer an oversize box cutter with break-off tips to get a new cutting point. Press down on the straight edge and cut through both sheets with a single stroke.

It’s just about impossible to cut two perfectly straight lines but you can cut two curves that exactly fit together. Overlap the two sheets you want to join. Temporally tape them down to your cutting table except the edge you are going to cut. Use a straight edge to flatten the cut edges together and use a very sharp knife. I prefer an oversize box cutter with break-off tips to get a new cutting point. Press down on the straight edge and cut through both sheets with a single stroke.

This works well. If a curvy line would serve your art, you can cut a freehand line this way also. In either case, the key is to cut both pieces at once and ensure that nothing moves while you’re doing it.

Why use a curvy line? To make it a feature, not a bug!

CK =)
I take great comfort in knowing that my genuine typos will probably be blamed on some device's autocorrect.
DIY art supplies, sketches, and more: cyntada.com / @cyntada

Hey… I was talking about a steel ruler and a craft knife. You can get both from the hardware stores at a very affordable price.
My 1 meter steel ruler cost me 6 euros and the large craft knife another 2.

One problem that you likely won’t be able to overcome even if the joint is absolutely perfect – paint will soak in along the cut edges, penetrating into the body of the paper, producing darker, visible lines at the junction.

You could seal the paper with a clear acrylic medium and paint on the patchwork panel with oils or acrylics and possibly be able to hide the joints.

One problem that you likely won’t be able to overcome even if the joint is absolutely perfect – paint will soak in along the cut edges, penetrating into the body of the paper, producing darker, visible lines at the junction.

You could seal the paper with a clear acrylic medium and paint on the patchwork panel with oils or acrylics and possibly be able to hide the joints.

If the papers are properly glued with the acrylic medium on the boards this will not happen. The trick is to apply the acrylic medium ( that is better than any glue) evenly. The best way to do this is by using a plastic cart ( like the credit cards) and spread the medium like butter on the board. As evenly as possible.

Then you place the paper and you press it down either with your hands ( that has to be clean) or with a soft cloth from the middle of the paper to the edges.
The excess acrylic medium on the other side of the board that you are going to glue the other paper you spread it further with the card and the rest you use it to seal the edges of the paper.

If the papers are straight and properly placed on the board the joints will not be visible.

I think Marialena, you are talking about mounting the piece after it has been painted?

I have tried many times to join two pieces together and no matter how good of a join you have the water+paint will always behave differently around the edge and the joint will always be visible. Unless you seal the paper, but then, why use watercolor paper at all?

No before. You have to mount the paper before you paint it. Back in the old days that we didn’t have illustration boards and there were not available very thick papers ( above 600 gsm that are actually thick as illustration boards) we did that very often ( and not even with acrylic medium that wasn’t available either back then).

The edges will not be affected if you spread the acrylic medium evenly on the board and you press down the paper from its center to the edges.

You’ve got to be careful though not to scratch or spoil the surface of the paper with the medium, while you push it down to stick on the board.

I used to use a roller like this one, with a thick elastic cylinder, one of those what we used back then to spread ink on the press machines ( clean not with ink! :lol:)

https://sc02.alicdn.com/kf/HTB1zQkVj3DD8KJjy0Fdq6AjvXXap/JSY920-Neoprene-roller-Aluminum-Tube-Rubber-Vinyl.jpg

over a piece of transparent paper that I placed it over the surface of the paper.

Let me repeat the steps for doing so.
Use a plastic card to spread the acrylic medium on the board. The area of each paper separately.
Then place the paper on top.
Press it down from the middle to the edges either with your clean hands, or a roller, or with a soft clothe ( protect the surface of the paper somehow. Don’t scratch it with your nails).

Use the excess medium that is in the edges to seal the edges. Spread the excess medium in the middle to the other side of the board. Apply more acrylic medium. Place the next paper and do all the same.

At the end place the board flat under some weight, ( books or something ) until it dries.
If you have a press even better.

https://koutsipetsidis.files.wordpress.com/2016/06/dimitris-bookbinding-corner-modified-and-restored-nipping-press.jpg?w=249&h=319

If you don’t put another board over the transparent paper that is over the paper, stuck on this something heavy and leave it there for a couple of hours.

It is easier and less complicated than it sounds.