In The Spongebob Squarepants Movie (2004) we see at one point during the final song a guitar solo wich we see a non cartoon guitar for a couple seconds. So that mean
SpongeBob Electric Guitar Outfit | Reverb
A children's Spongebob Squarepants Viacom International six-string acoustic guitar, in vibrant yellow with images of Spongebob Squarepants and one smaller ten-stringed instrument (2). | Adam Partridge