When she was finally happy with the results, she left her job as a domestic maid and convinced her husband to sell the VW Beetle he used as a taxi. They invested $1,500 (__965; 1,206 euros) to open a small beauty parlour in 1993.
Long queues were soon visible outside the salon, in an old house in a poor neighbourhood in Rio de Janeiro. The doors would open at 8am, but women were already queuing three hours earlier.
"We were offering something innovative that didn't exist in the market," says Mrs Assis, known simply as Zica, today a successful 51 year-old businesswoman.
Zica came up with a formula for a treatment, enriched with nutrients and moisturising ingredients such as cocoa and acai extract.
She swings her head and sways her hair to explain what it is all about: the formula changes very frizzy hair - which sometimes naturally forms an "afro" if left untreated - into loose, bouncy locks which tumble downwards, all without the need for hair-straightening products.
Half of Brazil's population is black or mixed race, but the beauty industry was mainly focused on the ones of European descent. Zica and the three partners who joined her found a hungry market.
That was the beginning of Beleza Natural - Portuguese for "natural beauty". Today, the company has 12 shops in Rio, Bahia and Espirito Santo. It started with four employees. Now there are more than 1,400.
It also has a training centre for staff and a factory that makes 250 tonnes of hair products each month. Shampoos, conditioners and creams all come in bottles with pictures of smiling women with the brand's trademark curly hair.
Although the treatment has a strong appeal to people of African descent, Zica does not define her target market by race. Whether black or white, she points out that three-quarters of Brazilians have frizzy or curly hair.
"The market simply didn't see this before me! People with curly hair were forgotten."
Zica comes from a poor family of 13 children and grew up in a favela in Rio. When she was nine years old, she began to work as a maid and cleaning lady to help bring money home.
She remembers growing self-conscious of her hair's volume and fearful that it might cause a bad impression in richer people's houses.
Back then, the only alternative to her Afro look was to straighten her hair with chemical products. As a teenager, she felt obliged to do it.
"I wasn't happy about it, but I kept straightening my hair until I was 21. Then I said - enough. I want to wear my hair naturally."
Zica enrolled in a hairdressing course offered by a church in the favela where she lived. And so her research began.
Today, she has three children and lives in a gated community in an upmarket area in Rio. She is an exception in a country that still counts few black people among their rich.
"We came up from the bottom"
Each month, 80,000 people head to her salons for the treatment. It costs $35 (__22; 28 euros) and there are special packages for families.
Most of her clients are working-class women aged 18 to 45. Zica says her major concern is keeping prices affordable.
"We came up from the bottom, we know what people go through. I could never take care of my hair in a salon. I haven't forgotten this reality."
Her greatest reward, she says, is seeing her clients' self-esteem rocket as they grow prouder of their hair. "Every day I meet people with great stories to tell of the changes they went through," she says.
Although her company found an important niche, the early days were not easy. Zica and her partners had to battle Brazilian bureaucracy and face the mistrust of bankers.
"When you're an entrepreneur you dream a lot and you think people are dreaming with you. And actually that's not how things are. If you go to a bank and ask for a loan, people want proof. For a long time, we had to work on one day to pay on the next."
In 2005, the company won the support of Endeavor, an international NGO that helps promising start-ups develop business plans.
In 2012, their aim is to grow 25% on last year's business. The company has plans to expand further in Brazil - and dreams of going abroad.
"I'm sure we'll still go very far and conquer the world out there," says Zica. "I believe there are many people with frizzy hair waiting to be treated by us."
By BBC News