Many people of the 18th c. had imaginative spelling, as there were really no authoritative references. You can find some fun spellings in William Clark's journals ( of Lewis and Clark) , like "deckinsary" for dictionary, but Clark ( born in 1770) learned to read before Noah Webster's Spelling Book, America's first, came out in 1783. Samuel Johnson's first dictionary of English came out in 1755, after Washington had already learned to read and write.

But unlike Clark, Washington had a good education. A huge number of Washington's diaries, letters and even school exercises have been preserved and are available online at the Library of Congress. So, it is easily possible to look at his writing over his entire life, and though the orthography is period ( like "would" often written as "wou'd ) penmanship, spelling and grammar are, actually, pretty good and pretty consistent. He seems to have been taught with examples: you can look at his school exercises and see he was set to copy poems, moral essays and even legal documents ( he was training to be a surveyor) . Probably like a lot of students, he was taught by transcribing what his teacher thought to be good prose.

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