Panel issue with borders...so sad.
 

I've bought several really cute panels for little kids. They've been hiding in my UFO container so time to let them see the light of day and ready for Project Linus. I just add different rows of colors for borders. Makes a darling quilt. Sounds easy, right? Well, I've discovered I end up stretching the first border, so panel ends up bulging. So, take the borders off and find it's3/4" too short. ^%$$#&^% (Read as bad words... lol.) This happened on both ends. Now I have to figure out how I do this, so it won't happen again. Have a feeling I may have to measure the length and cut borders according. UGH!!!

Did you square up the panels before you put borders on? I've never had a panel yet that didn't need to be "straightened". Not sure if that's your problem. I dislike doing panels because most are so uneven. Hopefully, someone else will have better advice.

I find starching heavily helps with the stretching of fabrics as well as fraying. As for sashings and borders, I measure from both sides as well as the middle and take the average of the 3. This seems to keep the borders from wavy when I go to quilt it. I'm sure there's more ideas coming your way. I only have 1 UFO panel quilt and I designed it myself to go around the panel. I'm still trying to figure out how to get the sashings I'm adding between each row of blocks going around the panel but I think I know what I can do after hesitating for a mere 5-6 years.

I had a beautiful panel that was crooked and stretching didn't help much. I ended up making the borders into a wedge shape to get it square. I reversed the wedge shape on each side and straight on top and bottom. Then put a regular wider border around that. Looked great. Good enough to give to my friend's charity auction.

I'm getting ready to tackle a box full of donated panels. This suggestion sound like a great solution for any problems I encounter!!!

When I add borders to any quilt or panel, I always make it longer than it needs to be and then sew it on without stretching leaving excess at both ends, and then trim the excess length from both ends.

I would wash and dry the panels to see what they will do "on their own".

Sometimes fabric gets pulled off grain while it is being processed.

If the panel is printed off-grain - that's another deal.

One can pull it off- grain to make the panel look " right " -
​​​​Or
one can make design changes to the panel so one can use it on-grain.

Stay stitch around the panel with a 2.0 stitch length. Then press along the edge. Don't stretch or pull as you press. I also stay stitch long borders and then press. Pin the borders onto the panel and stitch.

I block the panel before using. Tape down one selvedge edge and one cut edge. I use my 12.5 inch square ruler to get a true 90 degree corner. Then spray to dampen and maneuver the other two edges into position. Measure each opposing edge to make sure they are the same length and also measure diagonally. Tape down and spray until wet. Leave to dry and then press. I have always had good luck with this method.