Source: Mike Flannagan (October 1, 1992) "TV Artist Bob Ross Watches Paint Dry, Turns It Into a Successful Career", The Knoxville News-Sentinel, p. B1.
“Take care for purity in the paint and not so stinky thick of grease, thin, thin, thin! And just on the light [parts in the painting] here and there a small push of thick [paint].... thick house-interiors are unpleasant - long drawing before you start and arrange pleasantly together all things before you start to paint - if the money does not bother you, it is always useful to visit Rott. [Rotterdam].”
translation from the original Dutch: Fons Heijnsbroek
version in original Dutch (citaat van Jozef Israëls' brief, in het Nederlands): Zorg voor zuiverheid in de verf en niet zoo stinkerig dik van smeerderij, dun, dun, dun, en zo op het licht hier en daar een zetje dik[ke verf].. ..dikke binnenhuizen zijn onaangenaam - lang teekenen voor je begint en het prettig bij elkaar arrangeren voor gij aan het verwen gaat - als het geld u niet begroot, is het altijd nuttig om eens naar Rott. [Rotterdam!?] te gaan.
Quote of a letter by Jozef Israels to painter David de la Mar, 1867; as cited in Mythen van het Atelier, ed. Mayken Jonkman & Eva Geudeker; d'jonge Hond, Zwolle/The Hague, 2010 – ISBN 9789089102065 ( source online http://delamar.bntours.nl/!mad1832-bronnen.html)
Israels' painting technique did develop only rather slowly. In 1867 he still gave this rather traditional academic advice to the young painter nl:David de la Mar
Quotes of Jozef Israels, 1840 - 1870
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Jozef Israëls 33
Dutch painter 1824–1911Related quotes

OK who's going to identify that?
The Guardian, Saturday 26 April 2008

"Supermodel" (1999-11-29), from attilathestockbroker.com http://www.attilathestockbroker.com. Retrieved 2007-03-26.

“Beware not to get caught up in the thick of thin things.”
second side of the first tape
1975 - 1992, Oral history interview with Joan Mitchell, 1986
Source: Barbara Rose, Lee Krasner, Museum of Fine Arts, Houston (1983) Lee Krasner: a retrospective. p. 134.

“Faults become thick when love is thin.”
Quotations from Gurudev’s teachings, Chinmya Mission Chicago

“I must follow him through thick and thin.”
Source: Don Quixote de la Mancha (1605–1615), Part II (1615), Book III, Ch. 33.

“Through thick and thin, both over hill and plain.”
Second Week, Fourth Day, Book iv. Compare: "Through thick and thin, both over bank and bush", Edmund Spenser, Faerie Queene, Book iii, Canto i, Stanza 17.
La Seconde Semaine (1584)

version in original Dutch: Begin maar, je palet helpt je wel voort. (als je tobde over een schilderij of ertegenop zag om te beginnen)
Quoted by Maria Bilders-van Bosse, in her letter to A.C. Loffelt, 23 June 1895; from an excerpt of this letter https://rkd.nl/nl/explore/excerpts/763 in RKD-Archive, The Hague
his usual reaction if you worried about a painting or dread to start painting
posthumous quotes