It would only be understandable in as much as many Germans have heard English speakers making this mistake before as it is most definitely a mistake. Unfortunately it can take a long time to instinctively know which to use.
Here we use kennen as there is an accusative noun that the handyman knows. If it is a fact that he knows then use wissen eg "Der Heimwerker weiss, dass Bäume grün sind." -"The handyman knows that trees are green"
So the way I use to distingish whether I should use kennen or wissen is that I replace "know" with a different phase:
Kennen - acquainted with Wissen - know for a fact
I'm sure these probably aren't perfect and someone can find an example to disprove my method. However, these have seem to work for me so far.