Should I paint or seal a balsa bridge?
Paint and sealer add weight and may break the rules. When it can make sense, what it does to the mass, and why moisture is the real issue.
Updated · 1 min read
Short answer: usually no. Paint and sealers add mass that counts against you, and many contests restrict coatings. Check the rules first.
Weight
A coat of paint weighs far more than you'd expect on a bridge that weighs only 10 to 20 grams. Weigh a stick before and after a test coat to see.
Rules
Some contests allow only plain wood and glue, and some forbid paint or require unfinished sticks. Read the materials section. See reading your rulebook.
Moisture
Balsa absorbs and gives off moisture, which changes its weight and strength. Estimates suggest about a 10% property change across ordinary humidity. A coating slows this but does not stop it, and the extra weight is usually more costly. A better answer is to store the bridge in a dry, stable place and weigh it close to test day. See humidity and balsa.
When it might make sense
A bridge that must also look good for display, in a contest that allows a finish and doesn't weigh the coating, or a classroom model not scored on mass.